1899] THE SCOPE OF NATURAL SELECTION 193 



would arrive at maturity, and, so long as each tissue remained pro- 

 portionately active, health would result, but when this balance failed 

 degeneration and disease would result. 



We come now to the concluding question, the relation that 

 germinal development bears to somatic 



As an organism reaches maturity, the phenomena associated with 

 reproduction become manifest ; this fact is practically universal, it 

 holds good for multicellular and unicellular organisms alike, and for 

 both the animal and vegetable kingdoms. In unicellular organisms, 

 as we have seen, it is probable that there is a mechanical limit to the 

 size of the cell, beyond which growth as a single cell becomes 

 impossible ; this growth limit will not be the same under all conditions, 

 but must ultimately be reached in all forms of single-celled organisms. 

 In the metaphyta, under suitable conditions, there appears to be a 

 nearly constant tendency to growth at any place where a breach of 

 continuity is formed in a living tissue or tissues ; in the lower forms 

 of metazoa removal of a portion of tissue is nearly always followed by 

 growth of the remaining, so that more or less complete repair results ; 

 in the higher animals, on the other hand, this local reparative process 

 is much less complete, yet even here some attempt is always present. 



The fact that removal of tissue tends to produce activity and 

 growth at the seat of injury suggests that possibly some mechanical 

 limit to growth is one of the causes of cessation of growth. 



The inferences so far necessary to determine the relation that 

 somatic development bears to germinal may now be summarised as 

 follows. I have endeavoured to point out that facts do not favour 

 direct climatic modification, and I accept the Neo-Darwinian con- 

 clusion and believe that there is very little evidence for the trans- 

 mission of somatic responses. From a study of facts which have 

 universal applications I have endeavoured to show (1) that growth 

 and reproduction are in some way closely related ; (2) that facts justify 

 the inference that an increasingly complex food sequence prepares the 

 way for morphological quantitative specialisation ; (3) that some 

 morphological explanation of heredity is necessary to explain the facts. 

 Some such provisional theory as the following would, I believe, 

 explain the facts of heredity, growth, decay, and certain facts which 

 have reference to disease, better than previous theories : — 



1. That there is a mechanical nutritional limit of growth for each 

 cell, that this bulk limit varies according to physical conditions and 

 food supply, but is reached sooner or later by all growing cells 

 (Spencer). When this limit is reached, cell division takes place, 

 which may be equal, as in fission, or unequal as in budding, etc, 



2. Under conditions which demand variability of the organism, 

 conjugation of similar organisms placed under similar conditions would 

 be favourable for the attainment of this requisite variability. If 

 protoplasm is never directly modified by climatic conditions, then the 



13 NAT. SC. VOL. XV. NO. 91. 



