162 VEP.TEBRATE ZOOLOGY 



these embryos differentiate definitive structures or take on hard in- 

 tegumentary coverings, for they Uve at best for a few weeks and do 

 not acquire the adult characters. 



But what in nature corresponds to the growth-retarding agents 

 used in the laboratoryf The inhibitors that are responsible for 

 racial retardation or racial senescence, which are the same, are in- 

 ternal, and are probably associated with an aging of the heredity 

 chromatin or specific germinal protoplasm. The metabolic rate of 

 the germinal elements is believed to lose momentum from generation 

 to generation and from age to age, unless rejuvenated or secondarily 

 speeded up in some way. In the senescent species we must conclude 

 that the progressive slow-down of the germinal metabolic rate is ir- 

 reversible and that these forms must become extinct when they have 

 gone to the limit of their differentiational excesses. The generalized 

 forms may be looked upon as racially perpetually young or ready for 

 any new processes of differentiation. Whether the inhibiting agents 

 are external or internal the sanie types of morphological distortion 

 of the generalized condition result. 



Some of the specific cases of abnormal structure among present- 

 day fishes may be more directly attributable to the external condi- 

 tions under which they live and develop. What more reasonable 

 explanation of the bUnd cave fishes is there than that their embryos 

 have been victims of growth-depressing agencies, such as cold, low 

 oxygen content of the water, or darkness? Similarly many of the 

 abysmal fishes could be explained as the result of the unfavorable 

 developmental conditions of the sea depths (Fig. 74) . These creatures 

 are for the most part either eel-like forms or Head-Fish forms, the first 

 of which may be attributed to differential inhibition, and the second, 

 to general inhibition followed by differential recovery of apical struc- 

 tures. 



Similarly, racial senescence, as expressed in pffidogenetic forms, may 

 mean that certain species have been so slowed down in their develop- 

 mental momentum that they are unable to push their developmental 

 processes past a larval period, but that the germinal elements, that 

 belong to the part of the axis with the lowest rate of metabolism, go 

 on and become mature, thus enabhng these creatures to reproduce 

 while the somatic structures are still larval or juvenile in character. 



High, compressed forms appear to be the result of differential in- 

 hibition of the primary axis followed by differential recovery of the 



