PRESENT ASPECTS OF EVOLUTION 285 



again would be useless unless accompanied by in- 

 creased complexity and more or less readjustment of 

 the cells and fibrils of the nerve-centres. And all 

 these additions to, and readjustments of, the nerve- 

 centres must take place without any disturbance of the 

 other necessary adjustments already attained. This 

 is no simple problem. 



We will here neglect the fact that many other 

 changes are going on simultaneously. Legs are be- 

 ing formed or moulded into jaws, the anterior segments 

 are fusing into a head, and their ganglia into a brain ; 

 an external skeleton is developing. Furthermore the 

 increase of the muscular and nervous systems must be 

 accompanied by increased powers of digestion, respira- 

 tion, and excretion. Practically the whole body is 

 being recast. We insist only on the necessity of simul- 

 taneous and parallel changes in muscles, nerves, and 

 nerve-centres ; though what is true of these is true, in 

 greater or less degree, of all the other organs. 



You may answer that this is to be explained by the 

 law of correlation of organs ; that when changes in one 

 organ demand corresponding changes in another, these 

 two change similarly and more or less at the same 

 time and rate. But this is evidently not an explana- 

 tion but a restatement of the fact. The question re- 

 mains, What makes the organs vary simultaneously so 

 as to always correspond to each other? The whole 

 series of changes must to some extent be effected at 

 once and in the same individual, if it is to be pre- 

 served by natural selection. Fortuitous variations 

 here and there along the line of the series are of little 

 or no avail. That the whole series of variations 

 should happen to occur in one animal is altogether 



