296 NATURAL SCIENCE. Oct.. 



changes the cells in different places find themselves in very different 

 ■circumstances ; those lining tubes and hollows inside the body are 

 brought in contact with very different external forces from those 

 lining the outer side of the body. In correspondence with these 

 varied stimuli the organic material, the same in all the cells, responds 

 in different ways, producing cell-specialisation. 



In the epigenetic modification the next process which helps to 

 produce the specialisation of the organic body is of a physiological 

 nature. Every cell is an elementary organism, endowed with the 

 ■capacities of elementary organisms. But the cells resulting from the 

 division of the egg to a certain extent lose their individuality 

 in the new individuality of the whole. The purely morpho- 

 logical method of regarding cells tends to exaggerate their individual 

 importance. Although the whole shape and form of the organism is 

 produced by cell-multiplication, yet, considered as an organism, the 

 multicellular adult is better regarded as a continuous mass of an organic 

 material. Hertwig details many of the facts showing the intimate 

 correlations which exist between different parts of organisms, and 

 ■concludes that many of the elaborations of development must be set 

 down to the observed fact of correlative growth. This part of his 

 argument is perhaps more conclusive in so far as it is a criticism of 

 Weismann's doctrine of the existence of associated determinant to 

 account for correlations, than it is in itself satisfactory. Weismann 

 may have failed completely in giving a plausible explanation, or he 

 may have underrated the importance of correlation as a factor in 

 growth. Hertwig simply associates the two observed facts of the 

 physiological unity of organisms and the existence of correlated 

 growths. 



He elaborates at some length his view that different external 

 influences cause the development of identical incipia into different 

 final results. In course of this, he reviews the controversy between 

 Spencer and Weismann relating to the polymorphism of social insects, 

 and concludes with Spencer against Weismann that Natural Selection 

 is not the factor at work, but that the different environments of the 

 different forms have produced the divergent modifications. He sees 

 in it the same phenomenon as the production of roots from stems, or 

 of the upper part of a hydroid from the cut lower part. It is one of 

 the results of the identity of the organic matter all through the cells 

 and tissues of an individual, and of the capacity of this identical 

 matter to respond in varied ways to varied stimuli. 



The three great points of his theory are these. First, all 

 organisms possess a specific plasm of exceedingly complicated nature. 

 Secondly, the sexual cell, which is the origin of a new organism, 

 possesses a share of this plasm, and, being an elementary organism, 

 is able to grow and multiply. In this process of multiplication, the 

 resulting cells, formed as they are by equal-heired division, receive 

 each an unmodified share of the initial plasm, which, like the cells 



