73 SECTIONAL ADDRESSES 



Recapitulation in Colonial Organisms. 



A third body of facts which testify to the reality of the principle of 

 recapitulation is yielded by colonial forms. This was concisely expressed 

 by Lang in 1921 when, referring to a preceding discussion of criteria of 

 relationship, he wrote, ' It was noticed that the colony, like the individual, 

 exhibits growth stages of its own which recapitulate ancestral characters.' 

 In illustration of this reference may be made to the polyzoon genus 

 Stomatopora in which he (1907) traces the evolution of the series of forms 

 from S. antiqua and S. gregoryi of the Lower Lias (Sinemurian and 

 Charmouthian respectively) to S. smithi, of the Cornbrash. In this 

 series the angle between the branches at each dichotomy exhibits a pro- 

 gressive diminution from 180° in the earlier to 60° in the latest forms. 

 In the development of successive types acceleration leads to the gradual 

 elimination of the larger angles from earlier growth stages. The individual 

 chambers or zooecia exhibit a like phenomenon. In these the evolutionary 

 change of form is from cylindrical or very slightly pyriform to markedly 

 pyriform. Here again in the development of the later species accelera- 

 tion of the quite pyriform stage leads ultimately to the elimination of the 

 cylindrical zooecium from the early stages of growth. 



Summary. 



Since the question of recapitulation, more especially of adult characters, 

 has been the focus of controversy, it has been necessary, in order to 

 secure its reinstatement in its proper place as an established principle of 

 evolutionary thought, to deal with it at considerable length. It may be 

 useful, therefore, before passing on to the next part of our subject, to 

 summarise briefly the results of our discussion. 



The divers shades of meaning which have been attached to the term 

 ' recapitulation ' by various authors in recent years reflect the phases which 

 the phenomenon exhibits during the development of diflFerent organisms. 

 In ontogeny two main classes of features have been recognised, viz. 

 adult and juvenile, both of which undergo evolution in phylogeny. 



Recapitulation of adult features occurs more especially during the 

 neanic phases of development. It may be specific and complete, that is 

 to say it repeats the combination of features exhibited by geologically 

 recent adult ancestors. Cases in which this is manifested, even in some 

 detail, appear to be limited to the lower and simpler grades of animal life. 

 Adult recapitulation may also be specific and limited. That is to say it 

 may repeat only a few of the features of the adult ancestor. In this case 

 the features are not necessarily in correct combination shown by the adult 

 ancestral species, they may indeed be drawn from several such types. 

 Cases of this kind occur throughout the animal kingdom and include 

 many well-known examples, such as the teeth in the young Ornitho- 

 rhynchus, the three centres of ossification in the avian metatarsal, the 

 horny claws on the wing digits of the un hatched duck. 



Adult recapitulation is exhibited also during the growth of meta- 

 merically repeated parts, of colonial forms and rejuvenated individuals. 



