ORGANIC DIFFERENTIATION yy 



plasm, and regarded it as the sole depository of the forces 

 regulating the evolution of organisms. The other substance, 

 the soma or body-plasm, although distributed throughout 

 the whole body, he considered as a protection provided 

 for the needs of the germ-plasm, to protect it against 

 the action of the external environment, to whose in- 

 fluences alone it yields. It is quite evident that such a 

 conception is the absolute negation of every scientific 

 explanation of living organisms, and it is indeed strange that 

 no one should have perceived how the facts upon winch it 

 rests, far from serving as a starting-point for a general theory 

 of evolution, were actually the specific result of a modification 

 of embryogenetic processes concerning which we shall shortly 

 have more to say. We have more reason to dwell on the 

 important modifications that organisms exhibit under the 

 influence of numerous internal secretions and of certain 

 substances introduced from outside, such as the secretions of 

 a number of parasites x or the poison injected by the sting of 

 particular insects, 2 which have led us to postulate the existence 

 of special substances, the hormones, 3 which stimulate the 

 various organs to react upon one another at a distance, and 

 thus maintain the necessary solidarity within the organism. 

 The hormones and the parasitic secretions constitute the 

 mechanism of this reciprocal adaptation of organisms, the 

 importance and range of which I pointed out in 1881, in the 

 following terms : — 4 



" The direct causes for division of labour and the 

 modifications connected with it in the associated merids are 

 found in large measure, as in the case of the plastids, in the 

 social life itself. Whenever two or more organisms enter into 

 constant relations with each other, modifications of a more or 

 less important nature take place in each of them." 



In making use of the term social life, we come to the question 

 that dominates the entire evolution of living beings, namely, the 

 nature of that mechanism regulating the constitution of the long 

 series of organisms, which begins with the first minute living 

 things and culminates in types that are so far removed from the 

 original starting-point in size, form, complexity of structure, 



1 The wasps of the Terebrantia series, for example, producing the galls 

 of the corn blight, etc. 



1 Galls produced by Cynipidae. ■ XXIII and XXV. « XXVIII, 710. 



