ANIMAL PEDIGREES 225 



There is no doubt that abundance of food yolk is 

 a direct and very frequent cause of the omission of 

 ancestral stages from individual development ; but 

 it must not be viewed as the sole cause. It is quite 

 impossible that any animal, except perhaps in the 

 lowest zoological groups, should repeat all the 

 ancestral stages in the history of the race ; the 

 limits of time available for individual development 

 will not permit this. There is a tendency in all 

 animals towards condensation of the ancestral 

 history, towards striking a direct path from the egg 

 to the adult. This tendency is best marked in the 

 higher, the more complicated members of a group 

 i.e., in those which have a longer and more tortuous 

 pedigree ; and, although greatly strengthened by 

 the presence of food yolk in the egg, is apparently 

 not due to this in the first instance. 



Thus the simpler forms of Orbitolites, such as 

 O. tenm'ssima, repeat in their development all the 

 stages leading from a spiral to a discoidal shell ; 

 but in the more complicated species, as Dr. Car- 

 penter has pointed out, there is a tendency towards 

 precocious development of the adult characters, the 

 earlier stages being hurried over in a modified 

 form ; while in the most complex examples, as in 

 O. complanata,) the earlier spiral stages may be entirely 

 omitted, the shell acquiring almost from its earliest 

 commencement the discoidal mode of growth. There 

 is no question here of relative abundance of food 

 yolk, but merely of early or precocious appearance 

 of adult characters. 



Of causes other than food yolk, or only in- 



