Atavism and Variation in Crosses 99 



the conditions within the organism, become again favor- 

 able to further development. 



As a typical example of these arrests of development 

 may be mentioned the well known case of the aquatic 

 salamanders, (newts). These tailed amphibians at a cer- 

 tain stage of their ontogeny take to the land, lose their 

 gills, and become accustomed to respiration by lungs. If 

 however they are prevented from doing that by impris- 

 onment in a closed aquarium, they retain their gills, and 

 the triton is halted for life at a low stage of development, 

 which its near relatives, the perennibranchs never pass. 



The hypothesis of centroepigenesis, which has thus 

 been derived in its entirety from the fundamental bio- 

 genetic law taken in its first degree of approximation, that 

 is in the sense of an exact repetition of phylogeny by 

 ontogeny, implies also that in two species arising from a 

 common ancestor, the series of specific potential elements 

 remain the same up to the ontogenetic stage corre- 

 sponding to this common ancestor, and only after this 

 stage do the series of elements concerned in the two spe- 

 cies diverge from one another. 



It follows that in crosses development can go on very 

 well so long as the two series of germinal elements are 

 identical, but it becomes hindered as soon as the elements 

 concerned, which strive to become active at the same time, 

 thwart one another by their difference. And through this 

 hindrance to development the organism will take on a 

 form similar to that of the common ancestor. Further a 

 few germinal elements too feeble heretofore in relation to 

 the others, and therefore unable to become active during 

 the development of organisms of the pure strain, are able, 

 if common to both races, to acquire by their union a pre- 

 ponderance over the others different in the two species, 



