216 CAUSES OF VARIATION 



Control of internal causes affecting the race as a whole. 

 Whatever causes of this nature may be at work in our fields 

 and yards, and they are to be reckoned with, our control 

 over them is secondary and indirect. Their effects, if present, 

 are at once insidious and sweeping. 



We can be mindful of the effects of genetic selection and the 

 selective death rate, and provide against them, at least to a 

 large degree. If growth force and orthogenesis are also forces, 

 they can be assisted or held in check by selection, but they can 

 never be absolutely controlled ; and if germinal selection is a fact, 

 it is going on entirely independent of any control which present 

 knowledge enables us to exercise except through selection. 



Summary. All that is involved in heredity is contained in a 

 minute bit of living matter passed from parent to offspring, and 

 whose development will constitute the new individual. The im- 

 pulse to development, therefore, and its fundamental possibilities 

 are forces internal to the germ and to the living organism. 



It is not difficult to see many causes of variation in the 

 internal processes known to be involved in the activities of 

 living protoplasm. Growth is the result of cell division, which 

 seems to proceed upon plans calculated to insure qualitative 

 as well as quantitative equality as between the daughter cells. 

 Any deviation from the plan, however, and deviations are 

 known to occur, must result in variation. This is especially 

 true in the reduction process which is characteristic of matura- 

 tion in both sexes, and which probably lies at the basis of bud 

 variation and of many mutations. 



Fertilization and sexual union are processes calculated to 

 effect new combinations out of the elements involved, though 

 the possibilities in this direction would be rapidly reduced by 

 close breeding or by any other circumstance which simplifies 

 the ancestry. 



Doubtless the condition of the germ has some influence, but 

 it is not well understood. The phenomenon of xenia, or double 

 fertilization in certain plants, causes the seed coats to vary the 

 first year in the same direction as the germ. Telegony is a 

 myth, and intra-uterine influences are doubtless limited to those 

 of nutrition, except in cases of disaster. 



