PRESIDENTIAL ADDRESS. 21 



mouth parts of the caterpillar to the sucking proboscis of a moth 

 would inevitably lead to starvation. The difficulty is evaded by 

 retaining the external form and habits of one particular stage for an 

 unduly long period, so that the relations of the animal to the 

 surrounding environment remain unchanged, while internally prepara- 

 tions for the later stage are in progress. Cinderella and the princess 

 are equally possible entities, each being well adapted to her environment. 

 The exigencies of the situation do not permit, however, of a gradual 

 change from one to the other ; and the transformation, at least as 

 regards external appearance, must be abrupt. 



Kleinenberg has recently directed attention to cases in which the 

 larval and adult organs develop independently ; the larval nervous 

 system, for instance, aborting completely and forming no part of that 

 of the adult. I am not sure that I fully understand Kleinenberg's 

 argument, but it seems very possible that such cases, which are 

 probably far more numerous than is yet admitted, may be due to what 

 may be termed the telescoping of ancestral stages one with another, 

 which takes place in actual development, and may accordingly be 

 grouped under the head of developmental convenience. Undue 

 prolongation of an early ancestral stage, as in cases of abrupt meta- 

 morphosis, must involve modification, especially in the muscular and 

 nervous systems ; in such cases a telescoping of ancestral stages takes 

 place as we have seen, the adult being developed within the larva. 

 Such telescoping must distort the recapitulatory history, and the shape 

 of the larva and adult may differ widely, an independent origin of 

 organs, especially the muscular and nervous systems, may be acquired 

 secondarily. 



The stage in the development of Squilla, in which the three 

 posterior maxillipedes disappear completely, to reappear at a later stage 

 in a totally different form, is not to be interpreted as meaning that the 

 adult maxillipedes are entirely new structures unconnected historically 

 with those of the larva. Neither is the annual shedding of the antlers 

 of deer to be regarded as the repetition of an ancestral hornless con- 

 dition intercalated historically between successive stages provided with 

 antlers. In both cases the explanation is afforded by convenience, 

 whether of the embryo or adult. 



Many embryological modifications or distortions may he attributed 



