400 V. THE NATURE OF EVOLUTION 



Tiighly specific effect of the "virus." Presumably hormones may act 

 as helpmates of viruses, assisting the development of the specific 

 structure determined by the latter. As was already pointed out, 

 mammary cancer virus or leucosis virus can act as such only when 

 sexual hormones or related factors are present. At an early stage of 

 the individual development, hormone-like substances may assist the 

 function of organizer, which can be looked upon as a strong structure 

 comparable to a virus. 



During pregnancy, striking changes take place in the feature of 

 hormone secretions, especially leading to hypertrophy of the hypophysis 

 associated with its increased fimction, indicating the requirement of 

 unusually increased activity of hormones for the development of the 

 foetus. 



3. The Effect of Use and Disuse 



It is a familiar fact that during the life of an individual, new 

 characters can be acquired by use, and that disuse leads to the 

 disappearance of the characters. For example, the use of muscles 

 increases their development, while disuse results in the deficiency or 

 •even complete loss of the function. According to the Lamarckian 

 theory the characters thus acquired in an individual are inherited by 

 its offspring. Although the majority of authors claim that this theory 

 has no support from modern biology, the writer is able to provide 

 the theoretical basis to this doctrine. 



The development of a structure by use can be regarded as a sort 

 of the general phenomenon that the cells are increased in their acti- 

 vity upon the reception of stimuli, a phenomenon which in turn may 

 be based upon that the protein molecules may be activated by a proper 

 physical or chemical factor as a result of temporary liberation of free 

 polar groups as previously stated. The change in certain somatic 

 ■cells thus produced by use may be transmitted to the germ cell 

 through the "virus", as in the case of other structures changed by 

 environmental factors. There is, of course, no reason to suppose that 

 the change brought about by use is different from that induced by 

 usual environmental factors. The change induced by use must likewise 

 be reversible, and so disuse will result in its disappearance. 



Also in the development of the pattern induced by use, just as in 

 that caused by usual environmental factors, hormones appear to be 

 involved. As above stated a severe muscular work leads to the 

 increased corticoid-hormone secretion. It is a well-known observation 

 that cows may be brought into full lactation some time before partu- 



