Page images
PDF
EPUB
[ocr errors][ocr errors][ocr errors]
[ocr errors]

present perfect plan of construction, could have profited the progenitors of the hive-bee? I think the answer is not difficult: it is known that bees are often hard pressed' to get sufficient nectar; and I am informed by Mr. Tegetmeier that it has been experimentally found that no less than from twelve to fifteen pounds of dry sugar are consumed by a hive of bees for the secretion of each pound of wax; so that a prodigious quantity of >fluid nectar must be collected and consumed by the bees in a hive for the secretion of the wax necessary for the construction of their combs. Moreover, many bees have to remain idle for many days during the process of secretion. A large store of honey is indispensable to support allarge stock of bees during the winter; and the security of the hive is known mainly to depend on a large number of bees being supported. Hence the saving of wax by largely saving honey must be a most important element of success in any family of bees. Of course the success of any species of bee may be dependent on the number of its parasites or other enemies, or on quite distinct causes, and so be altogether independent of the quantity of honey which the bees could collect. But let us suppose that this latter circumstance determined, as it probably often does determine, the numbers of a humble-bee which could exist in a country; and let us further suppose (differently to what really is the case) that the community lived throughout the winter, and consequently required a store of honey: there can in this case be no doubt that it would be an advantage to our humble-bee, if a slight modification of her instinct led her to make her waxen cells near together, so as to intersect a little; for a wall in common, even in two adjoining cells, would save some little wax. Hence it would continually be more and more advantageous to our humble-bee, if she were to make her cells more and more regular, nearer together, and aggregated into a mass, like the cells of the Melipona; for in this case a large part of the bounding surface of each cell would serve to bound other cells, and much wax would be saved. Again, from the same cause, it would be advantageous to the Melipona, if she were to make her cells closer together, and more regular in every way than at present; for then, as we have seen, the spherical surfaces would wholly disappear, and would all be replaced by. plane surfaces; and the Melipona would make a comb as periect as that of the hivebee.

Beyond this stage of perfection in architecture, natural selection could not lead; for the comb of the hive-bee, as far as we can see, is absolutely perfect in economising wax.

Thus, as I believe, the most wonderful of all known instincts, that of the hive-bee, can be explained by natural selection having taken advantage of numerous, successive, slight modifications of simpler instincts ; natural selection having by slow degrees, more and more perfectly, led the bees to sweep equal spheres at a given distance from each other in a double layer, and to build up and excavate the wax along the planes of intersection. The bees, of course, no more knowing that they swept their spheres at one particular distance from each other, than they know what are the several angles of the hexagonal prisms and of the basal rhombic plates. The motive power of the process of natural selection having been economy of wax, together with cells of due strength, and of the proper size and shape for the larvae; that individual swarm which made the best cplls, and wasted least honey in the secretion of wax, having succeeded best, and having transmitted by inheritance their newly acquired economical instincts to new swarms, which in their turn will have had the best chance of succeeding in the struggle for existence.

It has been objected to the foregoing View on the origin of instinct that “ the variations of structure and of instinct must have been simultaneous and accurately adjusted to each other, as a modification in the one without an immediate corresponding change in the other would have been fatal.” The force of this objection seems entirely to rest on the assumption that the changes in both instinct and structure are abrupt. To take as an illustration the case of the larger titmouse (Parus major) alluded to in the last chapter: this bird often holds the seeds of the yew between its feet on a branch, and hammers away till it gets into the kernel. Now what special difficulty would there be in natural selection preserving each slight variation of beak, better and better adapted to break open seeds, until a beak was formed, as well constructed for this purpose as that of the nuthatch, at the same time that hereditary habit, or compulsion from the want of other food, or the preservation of chance variations of taste, made the bird more and more of a seed-eater ‘I In this case the beak is supposed to be slowly modified by natural selection, subsequently

[ocr errors][ocr errors]

to, but in accordance with, slowly changing habit; but let the feet of the titmouse vary and grow larger from correlation with the beak, or from ,any other unknown cause, and is it very improbable that such largerfeet might lead the bird to climb more and more until it acquired even the remarkable climbing instinct and capacity of the nuthatch ‘I In this case a gradual change of structure is supposed to lead to changed instinctive habits of life. To take one morecase: few instincts are more remarkable than that which leads the swift of the Eastern Islands to make its nest wholly of inspissated saliva. Some birds build‘their nests of mud, believed to be moistened with saliva; and one of the swifts of North America makes its nest (as I have seen)vof sticks agglutinated with saliva, and even with flakes of this substance. Is it then .very improbable that the natural selection of individual swifts, which secreted more and more saliva, should at last produce a species with instincts leading it to neglect other materials, and to make its nest exclusively of inspissated saliva ‘2 And so in other cases. It must be admitted that in many instances'we cannot conjecture whether instinct or structure has first slightly changed; nor can we conjecture by what gradations many instincts have been developed when they relate to organs (such as the mammary glands) on the first origin of which we know nothing. _

No doubt many instincts of very difficult explanation could be opposed to the theory of natural selection,— cases, in which we cannot see how an instinct could possibly have originated; cases, in which no intermediate gradations are known to exist; cases of instinct of apparently such trifling importance, that they could hardly have been acted on by natural selection; cases of instincts almost identically the same in animals so remote in the scale .of nature, that we cannot account for their similarity by inheritance from a commbn parent, and must therefore believe that they have been acquired by independent acts of natural selection. I will not here enter on these several cases, but will confine myself to one special difficulty, which at first appeared to me insuperable, and actually fatal to my whole theory. I allude to the neuters or sterile females in insect-communities: for these neuters often differ widely in instinct and in structure from both the males and fertile females, and yet, from being sterile, they cannot propagate their kind.

The 'subject well deserves to be discussed at great length, but I will here take only a single case, that of working or sterile ants.v How the workers have been rendered sterile is a difficulty ;’but not much greater than that of any other striking modification of structure; for it can be shown that some insects and other articulate animals in a state of nature occasionally becomelsterile; and if such insects had been‘social, and it had been profitable-to‘the community that‘a number should have been annually born capable of- Work, but incapable of procreation; I can see hoi'v'ery great difficulty in this being “effected by natural Selection.,- But I must pass over this preliminary difficulty. "'The' great difficulty lies in the working ants differing Widelyffr'om both the males and the fertile females in structure, as in the shape of the thorax and in being destitute 0‘! wings and sometimes of eyes, and in instinct.‘ As far as instinct alone is concerned, the prodigious difference in this respect between the workers and the perfect females, would have been better exemplified by the hive-bee. If a Working ant or other neuter insect had been an animal in the ordinary state, I should have unhesitatingly assumed' that all its characters had been ' slowly acquired through natural" selection; namely, by an individual having been born with Some slight profitable modification of structure, this being inherited by ‘its offspring, which again varied and were again selected, and’ so onwards. But with the working ant we have an insect differing greatly frOm its parents, yet absolutely sterile {'50 that it could never have transmitted successively acquired modifications of structure or instinct 'to its progeny.“ It may well be asked how is it possible to reconcile this case with theltheory of natural selection ‘2

First, let it beL remembered that we have innumerable instances, both in our domestic productions and in those in a's'tate- of nature, of all sorts of differences of structure which have become correlated to certain ages, and to either sex. ‘We have differences correlated not only to one sex, but to that shirt period alone when the reproductive system‘ is active, as in the nuptial plumage of many birds, and in the hooked jaWS of the male salmon. We have even slight differences in the horns of different breeds of cattle in relation to an artificially imperfect state of the male sex; for oxen of certain breeds have longer horns than in other breeds, in comparison with the horns of the bulls or cows of these same breeds. Hence I can see no

[ocr errors][ocr errors][ocr errors]

real difficulty in any character having become correlated with the sterile condition of certain members of insectcommunities : the difficulty lies in understanding how such correlated modifications of structure could have been slowly accumulated by natural selection. ,

This difficulty, though appearing insuperable, is lessened, or, as ,I believe, disappears, when it is remembered that selection may be applied to the family, as well as to the individual, and may thus gain the desired end. Thus, a well-flavoured vegetable is cooked, and the individual is destroyed; but the horticulturist sowsv seeds of the same family, and confidently expects to get nearly the same variety :- breeders of cattle wish the flesh and fat to be well marbled together; the animal has been slaughtered, but the breeder goes with confidence to the same stock. I have such faith in the powers of selection, that I do not doubt that a breed of cattle, always yielding oxen with extraordinarily long horns, could be slowly formed by carefully watching which individual bulls and cows, when matched, produced oxen with the longest horns; and yet no one ox could ever have propagated its kind. Thus I believe it has been with social insects : a slight modification of structure, or instinct, correlated with the sterile condition of certain members of the community, has been advantageous to the community: consequently the fertile males and females of the same community flourished, and transmitted to their fertile offspring a tendency to produce sterile members having the same modification. And I believe that this process has been repeated, until that prodigious amount of difference between the fertile and sterile females of the same species has been produced, which We see in many social insects.

But we have not as yet touched on the climax of the difficulty; namely, the fact that the neuters of several ants differ, not only from the fertile females and males, but from each other, sometimes to an almost incredible degree, and are thus divided into two or even three castes. The castes, moreover, do not generally graduate into each other, but are perfectly well defined ; being as distinct from each other as are any two species of the same genus, or rather as any two genera of the same family. Thus in Eciton, there are working and soldier neuters, with jaws and instincts extraordinarily different: in Cryptocerus, the workers of one caste alone carry a wonderful sort of shield on their heads, the use of which is quite unknown:

« PreviousContinue »