This, however, would be a very shallow criticism. The veriest tyro in natural science is aware that, if there is any truth at all in the general theory of descent, we are everywhere compelled to see that the conditions which determine the development of a species in any direction are always of a complex character. Why one species should remain constant through inconceivably enormous lapses of geological time, while others pass through a rich and varied history of upward change—why this should be so in any case we cannot say. We can only say, in general terms, that the conditions which in any case determine upward growth or stationary type are too numerous and complex to admit of our unravelling them in detail. Now, if this is the case even as between the structures of allied types—where there may be nothing to indicate the difference of the conditions which have led to the difference of results,—much more must it be the case between animals so unlike as a parrot and an ape. I think he would be a bold man who would affirm that even if the orang-outang had been able to articulate, this ape would necessarily, or probably, have become the progenitor of another human race. Absurd, then, it is to argue that, if the human race sprang from some other species of man-like creature, and became human in virtue of the power of articulation _plus_ all the other conditions external and internal, therefore the talking birds ought to have developed some similar progeny, merely because they happen to satisfy one of these conditions. Take a fair analogy. Flying is no doubt a very useful faculty to all animals which present it, and it is shown to be mechanically possible in animals so unlike one another as Insects, Reptiles, Birds, and Mammals. We might therefore suppose that, from the fact of bats being able to fly, many other mammals should have acquired the art. But, as they have not done so, we can only say that the reason is because the complex conditions leading to the growth of this faculty have been satisfied in the bats alone. Similarly “the flight of thought” is a most useful faculty, and it has only been developed in man. One of the conditions required for its development—power of articulation—occurs also in a few birds. But to argue from this that these birds ought to have developed the faculty of thought, would be just as unwarrantable as to argue that some other mammals ought to have developed the faculty of flight, seeing that they all present the most important of the needful conditions—to wit, bones and muscles actuated by nerves. Indeed, the argument would be even more unwarranted than this; for we can see plainly enough that the most important conditions required for the development of thought are of a psychological and social kind—those which are merely anatomical being but of secondary value, even though, as I have endeavoured to indicate, they are none the less indispensable.