'Activation of Last Element Ends Development 101 



and Darwin proves that this is a reversion to a blue flow- 

 ering ancestor. The tendency to incubate, which domes- 

 tic hens so often lose, always appears again in their 

 hybrids. The hybrids of ducks show a tendency to mi- 

 grate. The mule is harder to break thoroughly than the 

 horse or the ass. 67 



These examples afford, in our estimation, the most 

 certain proof that the ontogenetic stimuli of two species 

 arising from a common ancestor must remain alike dur- 

 ing a long series of earlier developmental stages, and only 

 in later stages begin to diverge from one another. And 

 this is just what the centroepigenetic hypothesis implies, 

 but what no other hypothesis has yet been able to explain. 



Further the hypothesis of centroepigenesis teaches us 

 that the series of like germinal elements must be shorter 

 the farther removed the species are from the common 

 ancestor. Now Morgan as is well known has obtained 

 hybrids in which, for example, eggs from Asteria were 

 fecundated by sperm from Arbacia, which belongs to the 

 genus Echinus. The two parent forms belong here not 

 only to two different genera but also to two different 

 classes. But these hybrids have never got beyond the 

 larval form, the pluteus which represents only one of the 

 first stages of ontogeny. 68 



The hypothesis of centroepigenesis, finally, regards 

 development as completed at the moment when all the 

 germinal elements have achieved activation. We note 

 that the central zone is then no longer required to employ 

 its acquisitions of nutritive material for the growth of its 



67 Darwin: Animals and Plants under Domestication. II. P. 13 

 21 : Crossing as a direct Cause of Reversion ; P. 254. 



"Morgan: Experimental Studies on Echinoderm Eggs. Anat 

 Anzeiger, Bd. IX. No. 5 and 6; Dec. 23, 1893. P. 151152. 



