478 THE GENERAL PRINCIPLES OF ZOOLOGY 





has a very noticeable effect on the rest of the body has long 

 been known, and animals have often been spayed or castrated 

 to bring about certain modifications. But in what way this 

 influence is produced is absolutely unknown, and so we have no 

 reason to assert that characters acquired by the somatic cells 

 cannot affect the germ-cells merely because we know no way of 

 bringing this about. In any case it is extremely probable that 

 the general nutrition of the body should affect the germ-cells as 

 well as the somatic cells, and we have in fact some evidence that 

 a lack of proper nutrition tends to the production of offspring 

 of a more atavistic type. 



Every germ-cell must be conceived of, however, as containing, 

 potentially at least, the energy necessary for bringing about the 

 various changes incident to development. But the actual struc- 

 ture of the germ-cell does not differ materially from that of many 

 other cells in the body. Just as in cell division the nucleus 

 assumes the prominent role, so in the germ-cell the nucleus 

 appears to be the more important part of the cell, and certain 

 experiments tend to show that it is a prominent factor in hered- 

 ity. Thus a sea-urchin egg may be broken up, and a portion 

 of the cytoplasm, which contains no part of the nucleus, allowed 

 to unite with a spermatozoon from another species of sea-urchin. 

 An embryo results, which possesses the characteristics of the 

 species from which the spermatozoon came. 



Concerning the whole matter of the inheritance of acquired 

 characters, the most that we can safely say is, that nothing is 

 proven either for or against it; a more comprehensive study of 

 facts which bear on this subject is much to be desired. 



As yet we have said nothing directly concerning the origin 

 of man. We have shown, however, zoologically that he is an 

 animal of the order Primates, suborder Anthropoidea, and 

 closely related structurally to the higher apes; and palaeonto- 

 logically we have noted a still closer relationship through the 

 intermediate type, Pithecanthropos erectus, of Java. The very 

 general belief among scientists to-day is to the effect that man 

 has developed by a natural process of evolution from a type 

 common to him and the higher apes; these differ too much 

 from one another and from man in certain details to admit of 

 any existing genus being considered the ancestor of man. But 



