190 ESSENTIALS OF BIOLOGY 



throughout a much wider habitat than that frequented by the 

 parent and for obtaining access to the host (if the organism is a 

 parasitic one), etc. Very often the meaning of discontinuous 

 development, on the selection point of view is, however, very 

 obscure. 



696. The Further Life-history. When development has 

 resulted in the appearance of an organism recognizably of the same 

 kind as the parent there may be a further juvenile phase when 

 parental care is still necessary to the well-being of the offspring. 

 This is obviously the case with the human infant, the young of 

 birds, etc. During the juvenile phase the young organism may 

 be fed by the parent, protected, trained, etc. Even its bodily and 

 mental development may still proceed. Even when the young 

 animal may live and feed independently of the parent it is still 

 reproductively immature. At some further phase there ensues 

 the full development of the essential and external reproductive 

 organs and the organism becomes able to emit ripe ova and 

 spermatozoa and to copulate, if its reproductive mode is the sexual 

 one. There follows then a more or less lengthy reproductive 

 phase in which we see the complete animal. Certain sexual, 

 bodily characters develop (hair on the face of the human male, 

 for instance). Development may be regarded as leading up to 

 this reproductive phase and when it comes to a pause the sene- 

 scence of the animal may be regarded as leading away from the 

 typical reproductive phase. In wild nature the animal usually dies 

 catastrophically and it must be rare, in any case, when indi- 

 vidual death happens solely as the result of " senile decay." 



69^. The Specificity of Developmental Phases. We may 

 summarize an indirect life-history, as for example, that of the 

 Barnacle, as follows : 



Ovum — > Embryogeny ending in the Nauplius Larva — > phase 

 of pause — > transformation to Cypris larva — > pause — > trans- 

 formation to fixed, definitive phase, the adult Bala?ius. 



And the history of a direct development, such as that of man, 

 may be summarized : 



Ovum — > Embryogeny ending in the development of the foetus 

 which is born as the infant — > juvenescent phase of continuous 

 growth, with some differentiation, ending in the sexually mature 

 individual at the age of puberty. It is customary to speak of 

 embryonic, larval, post-larval, fcetal stages, or phases, and the 



