THE CAUSATION OF DISEASE. 43 



evil effects of inter-breeding, which may not become manifest 

 for a long period of time. The good effects are shown in 

 increased constitutional vigour, while the evil results consist 

 in diminished size and vigour, and sometimes in actual malfor- 

 mation. Part of the evil effect, no doubt, must be attributed (in 

 the case of animal-organisms) to the fact that the same morbid 

 tendencies are apt to exist in parents of near blood, which 

 morbid tendencies thus have no chance of diminution in the 

 offspring'. But this will not account for the diminished size he 

 speaks of, far less for the tendency to malformation. 



It is very interesting to observe that much of the evil of 

 inter-breeding may be mitigated if some of the stock be kept 

 in different localities. The explanation is obvious. Differences 

 of E work differences in the S, and thus the inter-breeding- 

 animals are rendered more unlike. We may therefore lay it 

 down as proven that one great purpose which Nature has in 

 view in resorting to sexual reproduction is to secure the union of 

 protoplasmic masses derived from unlike organisms.* Inasmuch, 

 therefore, as distinct advantages accrue from sexual reproduction, 

 we can easily see how this method has become fixed by natural 

 selection. 



Can we push our reasoning farther, and answer the question : 

 How is it that close resemblance of the organisms brought 

 into sexual contact is inimical to proper reproduction ? 

 Darwin points out that changed conditions of life act bene- 

 ficially upon ?oii organism — we shall have occasion to allude to 

 this interesting fact again | — and he considers that the good 

 resulting from a cross may be due to the fact that each parent 

 has been subjected to different conditions. He seems to think 

 that in this way the more or less stable equilibrium of the- 

 germ and sperm is broken, and the formative forces thus 

 initiated. Herbert Spencer's hypothesis is more elaborate than 

 Darwin's, though in complete harmony with it. He asks the 



* Intercrossing not only benefits by sustaining the vigour of a race. It 

 further helps in this wise: "When species are rendered highly varied by 

 changed conditions of life, the free intercrossing of the varying individuals- 

 tends to keep each form fitted for its proper place in Nature, and crossing can> 

 be effected only by sexual generation." — Variation under Domestication^ 

 p. 355. See also Spencer's " Principle of Biology," vol. i. § 95. 



f See Part III. 



