THE CAUSATION OF DISEASE. 47 



virtue of which the developing cells attract to themselves the 

 proper gemmules, and in regard to this he says : " There are no 

 doubt ten thousand of Compositee, and if the pollen of all of 

 them were placed on the stigma of a particular species this 

 would elect ' with unerring certainty ' its proper pollen " 

 It is this second half of Darwin's hypothesis wherein the 

 great difficulty lies. Whether his way of putting the matter 

 is really an explanation is doubtful ; but there is no doubt 

 that from the beginning to the end of development the proper 

 aggregation of molecules is the result of affinities, and that 

 these are the outcome of material conditions. 



Herbert Spencer's doctrine is somewhat similar to Darwin's. 

 He, however, substitutes his " physiological units " for the 

 gemmules of Darwin.* 



Wherefore we may sum up the main facts of reproduction in 

 the four following propositions : — 



1 . Every cell of an organism has a tendency to reproduce 

 the whole organism. 



2. This power diminishes as specialization proceeds, becom- 

 ing more and more confined to special cells. 



3 . The cells capable of reproducing the entire organism may 

 consist of aggregated "gemmules," or "physiological units" 

 (using these terms in the sense their respective authors attach, 

 to them). 



4. The object of sexual reproduction is to secure the union 

 of reproductive cells from unlike organisms, so as to disturb the 

 equilibrium into which such reproductive cells tend to fall. 



Now, without further indulging in speculation, it is evident, 

 whether this, that, or the other hypothesis be correct, that the 

 whole mystery of reproduction or heredity hinges upon the 

 structure of the reproductive cell or cells. The difficulty lies 

 in the question — What determines the structure of the repro- 

 ductive element ? For, given a definite structure of germ and 

 sperm, there is no difficulty — I, at all events, see none — in 

 explaining, on the principle of causation already enunciated, 

 the development of the mature organism from them. When the 



* For a complete exposition of Darwin's doctrine of Pangenesis, see 

 " Variation under Domestication,'' vol. ii. chap, xxviii. See also Herbert 

 Spencer's " Principles of Biology," vol. i. chap. iv. a id viii. 



