y 2 Evolution and Adaptation 



two, three years, etc., that it does so because its ancestors 

 lived one year, two years, three years, etc. ? The descent 

 theory is correct so far as it maintains that older, simpler 

 forms have been the forefathers of later complicated forms. 

 In this case the resemblance of the older, simpler forms to 

 the embryos of later forms is explained without assuming 

 any law of inheritance whatsoever. The same resemblance 

 between the older and simpler adult forms, and the present 

 embryonic forms would even remain intelligible were there no 

 relation at all between them." 



Interesting and important as is this idea of His, it will not, 

 I think, be considered by most embryologists as giving an 

 adequate explanation of many facts that we now possess. It 

 expresses, no doubt, a part of the truth but not the whole 

 truth. 



We come now to a consideration of certain recently 

 ascertained facts that put, as I shall try to show, the whole 

 question of embryonic repetition in a new light. 



A minute and accurate study of the early stages of 

 division or cleavage of the egg of annelids has shown a 

 remarkable agreement throughout the group. The work of 

 E. B. Wilson on nereis, and on a number of other forms, as 

 well as the subsequent work of Mead, Child, and Treadwell 

 on other annelids, has shown resemblances in a large number 

 of details, involving some very complicated processes. 1 



Not only is the same method of cleavage found in most 

 annelids, but the same identical form of division is also pres- 

 ent in many of the mollusks, as shown especially by the work 

 of Conklin, Lillie, and Holmes. This resemblance has been 

 discussed at some length by those who have worked out these 

 results in the two groups. The general conclusion reached 

 by them is that the only possible interpretation of the 



1 On the other hand it should not pass unnoticed that Eisigh as shown in one 

 form (in which, however, the eggs are under special conditions being closely 

 packed together) that the usual type of cleavage is altered. 



