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.®7 
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.*®8 
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 
67Darwin: 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. 151—152. 
