chap, vi DIFFICULTIES AND OBJECTIONS 127 



grounds for the objection above made, that such small and 

 slight variations could be of no real use, and would not 

 determine the survival of the individuals possessing them. We 

 have seen, however, in our third chapter, that even Darwin's 

 terms were hardly justified; and that the variability of many im- 

 portant species is of considerable amount, and may very often 

 be properly described as large. As this is found to be the 

 case both in animals and plants, and in all their chief groups 

 and subdivisions, and also to apply to all the separate parts 

 and organs that have been compared, we must take it as 

 proved that the average amount of variability presents no 

 difficulty whatever in the way of the action of natural selection. 

 It may be here mentioned that, up to the time of the prepara- 

 tion 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 accu- 

 rate 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 in- 

 dividuals 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 



