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

mated for life, and this is agreat convenience to the fancier, for thus many races may be improved and kept true, though mingled in the same aviary ; and this circumstance must have largely favoured the formation of new breeds. Pigeons, I may add, can be propagated in great numbers and at a very quick rate, and inferior birds may be freely rejected, as when killed they serve for food. On the other hand, cats, from their nocturnal rambling habits, cannot be easily matched, and, although so much valued by women and children, we hardly ever see a distinct breed kept up; such breeds as we do sometimes see are almost a,l_ways imported from some other country. AlthoughLI do not doubt that some domestic animals vary less than others, yet the rarity or absence of distinct breeds of the cat, the donkey, peacock, goose, &c., may be attributed in main part to selection not having been brought into play :Jin cats, from the difficulty in pairing them: in donkeys, from only a few being kept by poor people, and little attention paid to their breeding; for recently in certain parts of Spain and of the United States this animal has been surprisingly modified and improved by careful selection: in peacocks, from not being very easily reared and a large stock not kept: in geese, from being valuable only for two purposes, food and feathers, and more especially from no pleasure having been felt in the display of distinct breeds; but the goose seems to have a singularly inflexible organisation.

To sum up on the origin of our domestic races of animals and plants. Ihelieve that the_c_:9_nditi_qns_o_f_lifeL from thegggiiflnpn the reproductivesystemLare 5015: of thejighesL importance, as causing variability. It is not probable that variability is an Inherent andhecessary contingency, under all circumstances, with all organic beings, as some authors have thought. The effects of variability are modified by various degrees of inheritance and of reversion. ' ariability is governed by many unknown laws, more especially by that of correlation of growth] Something may be attributed to the direct action of the conditions of life. Something must be attributed to use and disuse. [The final result is thus rendered infinitely complex. In some cases the intercrossing of species, aboriginally distinct, has probably played an important part in the origin of our domestic breeds,7 When in any country several domestic breeds have once been established, their occasional intercrossing,

[graphic]

with the aid of selection, has, no doubt largely aided in the formation of new sub-breeds; but [the importance of the crossing of varieties has been greatly exaggerated, both in regard to animals and to those plants which are propagated by seed] In plants which are temporarily pr0pagated by cuttings, buds, &c., the importance of the crossing both of distinct species and of varieties is immense; for the cultivator here quite disregards the extreme variability both of hybrids and mongrels, and the frequent sterility of hybrids; but the cases of plants not propagated by seed are of little im ortance to us, for their endurance is only temporary. Over all these causes of Change I am convinced that the accumulative action of Selection, whether applied methodically and more quickly, or unconsciously and more slowly, but

more efficiently, is by far the predominant Power?!

[ocr errors]

CHAPTER II

VARIATION UNDER NATURE

Variability — Individual differences — Doubtful species — Wide ranging, much diffused, and common species vary most—Species of the larger genera in any country vary more than the species of the smaller genera—Many of the species of the larger genera resemble varieties in being very closely, but unequally, related to each other, and in having restricted ranges.

BEFORE applying the principles arrived at in the last chapter to organic beings in a state of nature, we must briefly discuss whether these latter are subject to any variation. To treat this subject at all properly, a long catalogue of dry facts should be given; but these I shall reserve for my future work. Nor shall I here discuss the various definitions which have been given of the term species. No one definition has as yet satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species. Generally the term includes the unknown element of a distinct act of creation. The term “ variety ” is almost equally difficult to define; but here community 'of descent is almost universally implied, though it can rarely be proved. We have also what are called monstrosities ; but they graduate into varieties. By a monstrosity I presume is meant some considerable deviation of structure in one part, generally injurious to or not useful to the species. [Some authors use the term “variation” in a technical sense, as implying a modification directly due to the physical conditions of life; and “variations” in this sense are supposed not to be inherited: but who can say that the dwarfed condition of shells in the brackish waters of the Baltic, or dwarfed plants on Alpine summits, or the thicker fur of an animal from far northwards, would not in some cases be inherited for at least some few generations ‘2 and in this se I presume that the form Would be called a variety

It may perhaps be doubted wheth’er monstrosities, or such sudden and great deviations of structure as We occasionally see in our domestic productions, more

5O VARIATION UNDER NATURE

especially with plants, are ever permanently propagated

in a state of nature. Monsters are very apt to be sterile;

and almost every part of every organic being, at least

with animals, is so beautifully related to its complex

conditions of life that it seems as improbable that any

part should have been suddenly produced perfect, as that

a complex machine should have been invented by man

in a perfect state. I have not, at least, been able to

find good cases of species in a state of nature present~

ing modifications of structure resembling monstrosities

observed in allied forms. If such have occurred, their

perpetuation will have been due to their beneficial nature,

so that natural selection will have come into play. Many

cases are known of plants which regularly produce on

different branches, or on the circumference and in the

centre of umbels, &c., flowers of a widely different

structure; and if the plant ceased to produce flowers

of the one kind, a great change might perhaps suddenly

be effected in the specific character; but then we do not

at present know by what steps, or for what good, a plant

produces two kinds of flowers. With cultivated plants,

in the few cases known of a variety habitually bearing

flowers or fruit slightly different from each other, the

production of the variety has been sudden.

LAgain, we have many slight differences which may be

called individual differences, such as are known frequently

to appear in the offspring from the same parents,

or which may be presumed to have thus arisen, from

being frequently observed in the individuals 0 the same

species inhabiting the same confined locality._ No one

supposes that all the individuals jof the same speci€s are cast in the very same’i'nguld.“ kese individual dBfErerfce‘s

are highly impoFtaht formus, or they are often inherited,

as must be familiar to everyone; and thus they afford materials for natural selection to accumulate, in the

same manner as man accumulates in any given direction individual differences in his domesticated productions; These individual differences generally affect what naturalists consider unimportant parts; but I could show by a long catalogue of facts, that parts which must he called important, whether viewed under a physiological or classificatory point of view, sometimes vary in the individuals of the same species. I am convinced that the most experienced naturalist would be surprised at the number of the cases of variability, even

[graphic]
[graphic]

in important parts of structure, which he could collect on good authority, as I have collected, during a course of years. It should be remembered that systematists are_ far from pleased at finding variability in important characters, and that there are not many men who will laboriously examine internal and important organs, and compare them in many specimens of the same species. It would never have been expected that the branching of the main nerves close to the great central ganglion of an insect would have been variable in the same species; it might have been thought that changes of this nature could have been effected only by slow degrees ; yet quite recently Mr. Lubbock has shown a degree of variability in these main nerves in Coccus, which may almost be compared to the irregular branching of the stem\ of a tree. This philosophical naturalist, I may add, has also quite recently shown that the muscles in the larvae of certain insects are very far from uniform. [Authors sometimes argue in a circle when they state that important organs never vary; for these same authors practically rank that character as important (as some few naturalists have honestly confessed) which does not vary; and, under this point of view, no instanc of an important part varying will ever be found j but un er any other point of view many instances assuredly can be given.

There is one point connected with individual differences, which is extremely perplexing: I refer to those genera which have sometimes been called “protean” or “_p_ol_y~ mgrphic,” in which the species present an inordinate amourEI of variation; and hardly two naturalists can agree which forms to rank as species and which as varieties. We may instance Rubus, Rosa, and Hieracium amongst plants, several genera of insects, and several genera of Brachiopod shells. In most polymorphic genera some of the species have fixed and definite characters. Genera which are polymorphic in one country seem to be, with some few exceptions, polymorphic in other countries, and likewise, judging from Brachiopod shells, at former periods of time. These facts are very perplexing, for they seem to show that this kind of variability is independent of the conditions of life. I am inclined to suspect that we see in these polymorphic genera variations in points of structure which are of no service or disservice to the species, and which consequently have not been seized

[graphic]
« PreviousContinue »