WCP6645

[1] [p. 115]

[August 24, 1858]¹

I have been very much pleased with a paper in the last number of the Linnean Society's Proc[eedings]². on "the tendency of Species to form Varieties and on the Perpetuation of Varieties and Species by Natural means of selection,"³ by Darwin⁴ and Wallace. I am not quite sure that I altogether agree with them, but there is very much in it that is very good, and most of the ideas propounded are original. I think there is a hint in it on which you might speak, on the subject I suggested to you when at Castle Eden as being a likely one for a paper before the Linnean Society⁵, the variations induced by desert climate, as exemplified in North African Larks and Wheatears. The idea is perhaps not new, i.e. many naturalists know perfectly well that birds from desert localities do not exactly resemble individuals of the same species (i.e. good species, not those of bird-namers) from more favoured districts. Baird⁶ of Washington⁷ is quite familiar with this fact, and has or is about to put it into print together with the reasons whence he draws his conclusions. The great Gould⁸, too, has made remarks (Proc. Z.S., 1855, p. 78)⁹ bearing more or less on the same subject, with respect to the coloration of birds inhabiting forests and plains, sunny and cloudy atmospheres; but I do not suppose any one has connected these facts with the theory (though it is more than theory) of Darwin and Wallace, nor has any one practically applied their ideas. It seems to me that they can be connected and should be connected thus: any modification of the structure (using the word in its widest sense, even to comprehend [2] [p. 116] a mere change of colour) of an animal must in some way or other affect the ease or difficulty with which it con- contrives to maintain its existence. In the struggle for life which we know to be going on among all species, a very slight change for the better, such as improved means of escaping from its natural enemies (which would be the effect of an alteration in colour from one differing much to one closely resembling the hue of surrounding objects), would give that variety a great advantage over the typical or other forms of the species. Allow the advantage to be continued for a considerable period, and the variety becomes not only a race with its variations still more strongly imprinted upon it, but the typical form or varieties having experienced changes not advantageous to their life may even become extinct. Thus to apply the case, suppose an Algerian desert to become colonised by a few pairs of Crested Lark; we know that the probability is that of them one or two pairs would be likely to be of a darker complexion than the others, these and such of their offspring as most resembled them would become more liable to capture by their natural enemies, hawks, carnivorous beasts, etc.; the lighter coloured ones would enjoy more or less immunity from such attacks; let the state of things continue a few hundred years, the dark-coloured individuals would be exterminated, the lighter-coloured remain and inhabit the land.

Again, smaller or shorter-billed varieties would undergo comparative difficulty in finding food when food was not abundant, and had to be picked out from crevices among stones, these would be in comparatively reduced condition, in the breeding season they would not feel their capabilities were such as inclined them to matrimony, the consequences would be in a few hundred years the longer-billed varieties would be the most numerous, they would become a race, in a few hundred years more they would be the sole possessors of the land, the shorter-billed fellows dying out of their way until that race was extinct. Here are only two cases enumerated [3] [p. 117] which might serve to create, as it were, a new species from an old one, yet they are perfectly natural ones, and such as I think must occur, have occurred, and possibly be occurring still. We know so very little of the causes which, in by far the majority, if not in nearly all cases, make species rare or common, that there may be hundreds of others at work, some even more powerful than these, that go to perpetuate certain forms in Darwin's words according to natural means of selection. You may have a mere individual difference in the organs of digestion, and in this way produce a Gillaroo Trout with his gizzard-like stomach, out of a common Salmo fario. But for your paper you must first consult Darwin and Wallace, and you will understand that nothing that I have advised here is my own, but theirs, except the application of their theory to Algerian Larks and Irish trout. You should also get a little book of Vernon Wollaston's¹⁰ on the " Variation of Species,"¹¹ published a year or two ago by Van Voorst, the price of which is 5s¹². or so.¹³