4 DYNAMICS OF LIVING MATTER 



of the characteristics or determinants. If this view can be applied 

 generally, it is just as inconceivable that there should be gradual varia- 

 tions of an individual characteristic and intermediary stages between; 

 two elementary mutations, as that there should be gradual transitions 

 between one alcohol and its next neighbor in a chemical series. 



The fact that, as a rule, at a definite stage of development, larger 

 masses of sexual cells are formed, is one of the automatic phenomena 

 of development. The mechanism of this formation is unknown. 

 Miescher tried to solve this problem in the salmon. In this animal 

 the sexual cells seem to be formed at the expense of the substances 

 of the muscles, and it was the disappearance of the muscles at the time 

 the sexual organs began to grow which aroused Miescher's interest. 

 But this seems to be after all of only secondary importance, inasmuch 

 as with our present knowledge of the chemistry of living organisms 

 it is immaterial whether the animal's own muscles furnish the material 

 for the sexual cells or the muscles of the animal it devours. The real 

 problem is, how it happens that at a certain stage in the development 

 of the animal the sexual glands take away so much material from the 

 blood. From our present knowledge we must suspect that the mechan- 

 ism of such a process is a transformation of liquid constituents* of the 

 blood into solid constituents inside of those cells which show a rapid 

 growth or a transformation into different compounds. 



We possess a little more knowledge concerning sexual dimorphism. 

 It has been known for a long time that it is possible to produce in plant 

 hce (Aphis) either females exclusively, or both sexes, at desire. In 

 bees and related forms, as a rule, only males originate from unfertil- 

 ized eggs, and females only from fertiHzed eggs. It is known, more- 

 over, that in higher vertebrates such twins as originate from the same 

 eggs are also uniform in sex, while twins originating from different eggs 

 may be different in sex. All that we know thus far concerning the 

 origin of sex seems to indicate that the sex of the embryo is already 

 j determined in the unfertilized egg, or is determined very soon after the 

 'impregnation of the egg. 



Sexual maturity is sooner or later followed by death. Is death 

 determined just as automatically by the processes of development pre- 

 ceding it, e.g. the maturation of the sexual products, as are these pro- 

 cesses by the previous processes of development, or as is the develop- 

 ment of the egg by the entrance of a spermatozoon? The fact that 

 most higher animals, at least, die by bacterial infection, and that cer- 

 tain plants, e.g. the Sequoia, which are more free from bacteria, can 

 reach an almost fabulous age, renders the answer to this question some- 

 what uncertain or prevents the generalization of an answer. It is not 



