It may be here mentioned that, up to the time of the preparation of the last edition of _The Origin of Species_, Darwin had not seen the work of Mr.J.A.Allen of Harvard University (then only just published), which gave us the first body of accurate comparisons and measurements demonstrating this large amount of variability.

Since then evidence of this nature has been accumulating, and we are, therefore, now in a far better position to appreciate the facilities for natural selection, in this respect, than was Mr.Darwin himself.
Another objection of a similar nature is, that the chances are immensely against the right variation or combination of variations occurring just when required; and further, that no variation can be perpetuated that is not accompanied by several concomitant variations of dependent parts--greater length of a wing in a bird, for example, would be of little use if unaccompanied by increased volume or contractility of the muscles which move it.

This objection seemed a very strong one so long as it was supposed that variations occurred singly and at considerable intervals; but it ceases to have any weight now we know that they occur simultaneously in various parts of the organism, and also in a large proportion of the individuals which make up the species.

A considerable number of individuals will, therefore, every year possess the required combination of characters; and it may also be considered probable that when the two characters are such that they always _act_ together, there will be such a correlation between them that they will frequently _vary_ together.

But there is another consideration that seems to show that this coincident variation is not essential.