210 Proceedings of the Royal Physical Society. 



nature has laid herself out, in practically all forms of life, to 

 procure some sort of blend or cross at a time when the 

 powers of growth and division seem to be approaching an end. 

 To ask why this blend does cause a return seems a question 

 at present beyond our knowledge, but I have attempted to 

 frame a working hypothesis which would fit in with the above. 

 If life is to be looked upon as a rhythm of anabolism and 

 katabolism, as Professors Geddes and Thomson suggest, the 

 kind of anabolism must not only diverge in plants and 

 animals, but the form of this anabolism must further diverge 

 along the separate lines in which the diiferent groups of 

 animals and plants have evolved. In the individual, the 

 kind of anabolism must diverge along different channels to 

 form the different tissues, and it may be that not only do 

 the cells of the tissues have each their own power of building 

 up their own special form of tissue from a common food 

 material, but that they also may have a selective power on 

 the food supplied to them. It would seem, also, that during 

 specialisation the powers of simple anabolism or growth 

 without differentiation are gradually lost in this divergence, 

 until in some forms of tissue ordinary growth, as shown by 

 the power of cell division, is completely lost. To use the 

 idea suggested by Darwin's theory of pangenesis, if all these 

 different forms of anabolism found in the tissues of one 

 individual were conceived as being collected in a single cell, 

 it seems just possible that the different forms of anabolism, 

 as clearly shown in the different tissues, would be in some 

 degree antagonistic to one another, and become dormant or 

 potential in quality; while the powers of simple anabolism, 

 resulting in growth without differentiation, might be returned, 

 only to show once more the divergent forms of anabolism 

 when the effects of specialisation became marked. To apply 

 this to the germ cells, which have retained the power of 

 reproducing the animal in its entirety, these germ cells must 

 slightly diverge in their kind of anabolism in each individual, 

 or we should not have the phenomena of heredity so con- 

 stantly before us ; and it seems possible that a blend from 

 the cells of different individuals, wliich have diverged only 

 so far as can be seen by their results in heredity, may 



