12 PROBLEMS OF EVOLUTION 



divided into two, these two, instead of swimming off, each as 

 a separate individual, remained attached, then there would be 

 a two-celled animal. If this were continued further we should 

 have an animal composed of many cells in fact one of the Meta- 

 zoa. If they adhered together in a rounded mass there would 

 be an organism very similar to the Morula (see p. 1 6), that occurs 

 as one of the phases through which the embryo of any of the 

 higher animals passes. Supposing that differentiation of cells 

 arose, those on the outside, for instance, undertaking different 

 functions from those in the interior, there would be another 

 step onward in evolution. To this complex animal, whose life 

 depends on an unceasing process of cell-proliferation, there 

 might come a time of senility no less than to its one-celled 

 ancestor. And rejuvenescence might be possible only by a 

 process similar to the conjugation which gave to the ancestor 

 fresh life. But the cell which united with another cell, belong- 

 ing to another complex animal, would no longer be itself the 

 whole organism, but only one small part to which the duty of 

 reproduction was assigned, this small part a microcosm, in which 

 must lie dormant all the properties of the individual to which it 

 belongs, since from it a similar individual must spring. Thus 

 there would be no rejuvenescence of the whole organism, but 

 only of the cell set apart, which would thus, in its turn, be able 

 to develop into a complex animal. In fact, the repeated fissions 

 of the amoeba correspond to the repeated cell-divisions by means 

 of which a human being, for instance, grows : the myriads of cells 

 thus formed hold together and build up one organism, instead of 

 breaking away as independent individuals. 1 Sexual union corre- 

 sponds to the conjugation of the amoeba. But its vitalising effect 

 does not extend to the marvellously complex organism of which 

 the uniting cell has been a part. The vigour, due to the union, 

 goes to the formation of another highly organised individual closely 

 resembling those to which the uniting cells belonged. 



Among insects (aphides, for example) we find the curious 

 phenomenon of parthenogenesis, reproduction in which the male 

 takes no part, but though this may continue for many generations 



1 Mr Archdall Reid (Present Evolution of Man) explains this clearly. 



