THE EPIC OF EVOLUTION 



SOS 



Hurst (1870- ), on the other hand, thought of the lines represent- 

 ing the embryonic stages as divergent rather than parallel, since 



the farther back one traces r s*. ^l ^ 



1 . ., ccdxjtity stages 



the ontogeny, the greater the — ^ 



resemblance between different , am^y,i>^ian5^ , birds 



Imes. Morgan m reply sug- |i5>^s S^ reptlkS/ mammals 



gested that the reason for this "^ 



observation, indisputably true, 



may be because there are fewer 



available diagnostic features 



upon which to base differences 



the farther one goes back in 



development. 



Finally, O. Hertwig (1849- 



1922) has emphasized the fact 



that the different lines do not 



all start alike with the same 



egg stage. There are eggs and eggs. 



possibilities not attained by the fish egg. Thus, the eggs of the 



various groups, or "species-cells" as Hertwig calls them, have ac- 

 complished an evolution in themselves and attained different levels 



mammals *-*^ possibility, with the result that 

 the mammalian egg has a flying 

 start over the fish egg, and be- 

 comes in consequence an adult 



6 6 

 Morgan's " Repetition Theory." 



The mammalian egg has 



reptiles 



amphibians 

 fisV2es 



Divergence theory of Hurst. 



farther up the scale. This rela- 

 tion is shown in the figure. It is 

 evident that the lower adult type 

 of the fish can take no more than 

 a deceptively apparent part in 

 the developmental steps through 

 which the mammal passes. The 

 egg of the reptile, for example, in 

 a certain sense has reached a stage 

 of advancement, in possibilities 

 at least, somewhat comparable 

 with the adult attainment of the 



amphibian. The dotted lines in the figure show what would be 

 necessary to assume in order to picture how ontogeny repeats 

 phylogeny. 



H. w. H.— 33 



