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 he 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 adaptation, the joints and their ligaments being early approximated to their permanent conditions. Then the length of limbs seems to be effected as an embryonic adaptation. First take the case of man born with disproportionately short arms and legs. The legs have been interpreted 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 reduced 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 ancestral 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 differences as exist are seldom reminiscent.
