THE PALEONTOLOGIC RECORD 53 



In the matter of the succession of teeth the follicles which form the 

 last two the milk premolars form teeth in the first set of a totally 

 different and usually more advanced character than the teeth to be 

 formed from the same follicles in the permanent set. As a general 

 thing then the conclusion would be that the milk teeth tend to have 

 the^same characters as mark the permanent set, but when they vary 

 they often retain characters of the phylogenetically ancestral form. 

 Weber adds that the later the succession the less the difference between 

 the milk and permanent sets. 



Turning to the limbs, there are again several distinctly ontogenetic 

 characters, which are by no means ancestral. First, the formation of 

 epiphyses, so that a bone ossifies from three or more centers. This is 

 purely an ontogenetic adaptation and has no phylogenetic significance. 

 Then the articular ends of all the limb bones are greatly enlarged as 

 compared with adults. This again is not phylogenetic but an adapta- 

 tion, the joints and their ligaments being early approximated to their 

 permanent conditions. Then the length of limbs seems to be ef- 

 fected as an embryonic adaptation. First take the case of man born 

 with disproportionately short arms and legs. The legs have been inter- 

 preted as representing a phylogenetic condition, but the same rule does 

 not apply to the arms which were ancestrally long. This feature of 

 short limbs is also characteristic of carnivora and I feel that it is an 

 embryonic adaptation; certainly the ancestral limb can not be deduced 

 from the young condition. Quite the reverse of conditions obtains 

 among the Ungulata where the young at birth have disproportionately 

 long limbs, which with equal certainty does not represent any ancestral 

 condition recapitulated, for the ancestral limb in ancestral forms is 

 shorter. Again, I believe the anomalous legs are adaptations to either 

 the necessity for speed on the part of the young, or for height to reach 

 the teats, suckling being while the parent is standing. 



In the cases of the reduction of digits, greater portions of the re- 

 duced digits are usually found in the young animals than in the adults, 

 but in the case of the entire loss of a digit it is also lacking in the 

 young and embryo. 



The general conclusion of the whole matter would then be that the 

 young give us very little which is not deceptive in reconstructing an- 

 cestral forms. In certain cases, namely in the teeth and in reduction 

 of digits, confirmatory points may be obtained, but these must be 

 used with care, the valuable constructive evidence being rather found 

 in adult skeletons, and in morphological comparisons. While allowing 

 that many stages are recapitulated in the development of an individual, 

 the vast number of adaptations impressed on the young to be used after 

 birth, make their skeletons specialized even from birth, and such dif- 

 ferences as exist are seldom reminiscent. 



