THE STABILITY OF THE GERM-PLASM 95 



be grown in all the quarters of the globe, yet usually no appreci- 

 able change is observable. Even when a change is seen it is 

 generally a mere ' modification,' for the plant, if restored to its 

 ancestral environment, resumes the original form. ' Bud variations ' 

 occur, it is true, but they are comparatively very few, and we have 

 little reason to believe that they are due to the direct action of the 

 environment Thus, when a nectarine appears on a peach tree, 

 it is more likely that the change in the branch that bore it was 

 spontaneous than that it was caused by direct action ; for the 

 latter would imply that the environment of that one branch differed 

 amazingly from the environments of the other branches. 



153. The germ-plasm of the microbe is very complex ; but more 

 complex to an almost infinite degree is the germ-plasm of such a 

 being as man. The descendant cells of the fertilized ovum are not 

 mere copies of that ancestor, but members of a vast community in 

 which there is great diversity of form and function, and in which 

 every cell takes its appointed place and does its predestined work. 

 Presumably the orderly complexity of the body, with its immense 

 ramifications of large and small blood-vessels, lymphatics and 

 nerves, its organized groups of muscle, blood, gland, and nerve 

 cells, its skin, bone, and cartilage, indicates a corresponding 

 complexity of the germ-plasm. Yet, though this germ-plasm is 

 constantly exposed to all sorts of potent influences (e.g. toxins), and 

 has been so exposed for thousands of generations, so great is its 

 stability that all this great complex and even many developmental 

 processes which date back to millions of years before the evolution 

 of the vertebrate type are reproduced with unfailing accuracy by 

 every normal individual. Quite apart from the fact that Natural 

 Selection would have no scope if the hereditary tendencies drifted 

 this way or that at the mercy of the environment, it is evident that 

 no complex type could maintain the regular development and the 

 co-adaptation of its parts, unless the germ-plasm were endowed 

 with very great resisting power. 



1 54. Amongst multicellular organisms selection occurs in every 

 stage of development, amongst germ-cells and embryos as amongst 

 adults. Germ-cells, like unicellular organisms, are fitted to their 

 environments by characters which depend on the hereditary 

 tendencies carried by the germ-plasm. As is proved by the 

 variations of the individuals that develop from them, they vary 

 amongst themselves. They are in competition with one another, 

 some being better adapted for survival and the successful perform- 

 ance of their functions than their fellows ; some having more and 



