Physiology of Animals. 67 



or have formed the raw material of evolution remain 

 obscure. The literature of the pathology of tissues 

 and cells grows annually like an unending encyclo- 

 paedia. 



(5) The final step in pathological analysis leads, as in 

 physiology, to the study of the metabolism of proto- 

 plasm. For it is here that deranged function and 

 normal function have their foundation. As yet, how- 

 ever, the step has been, as it were, into the darkness, 

 with faint glimmerings of light which suggest the pos- 

 sibility of a new pathology and a new therapeutics. 



As a fine example of comparative pathological work 

 which is at the same time distinctively biological, we 

 may refer to MetschnikofFs epoch-making researches 

 on phagocytosis. 



There is as yet only a rudimentary physiology of 

 reproduction either as regards plants or animals. What 

 is called the physiology of reproduction is Reproduc . 

 usually a descriptive account of the pro- tion in 

 cesses by which eggs and male elements n 

 are formed, liberated, and brought together, and to 

 this there is usually appended some theory of the nature 

 of fertilization and the determination of sex. But the 

 descriptive account is only a needful preliminary, it 

 does not deal with the relation of reproduction to the 

 general metabolism of the body; and we are as far 

 from understanding the physiological meaning of fer- 

 tilization, or the conditions which lead one fertilized 

 ovum to become a male, and its neighbour to become 

 a female organism. Of theories there has been a pro- 

 fusion, and some of them may have a suggestive value, 

 but the majority of the earlier ones are mystical and 

 absurd, and the majority of the later ones hopelessly 

 partial. The Evolution of Sex (1889) contains a prelimi- 

 nary attempt to unify the various sets of phenomena 

 by restating them in terms of protoplasmic metabolism. 



Perhaps no scientific problem has been viewed with 

 more interest by outsiders than that of the determina- 

 tion of sex, that is, the analysis of the conditions which 

 determine whether an ovum shall develop into a male 

 or a female offspring. The interest is, of course, due 



