8o VARIABILITY 



plasm. I use the words ' direct ' and ' immediate ' because it is 

 conceivable that spontaneous variations are due indirectly and 

 remotely to the action of the environment, which, through Natural 

 Selection in the past, may have so dealt with the germ-plasm of 

 the species as to create a tendency to spontaneous variability. 1 



128. Normal offspring resemble their parents on the whole, 

 but differ from them in detail. We have to decide whether these 

 variations in details are spontaneous or caused by the direct influence 

 of the environment ', or, if by both, in what proportion, as a general 

 rule, the two kinds of variations occur. Now the offspring of 

 the same parents always vary, not only from both the parents, 

 but among themselves. Thus even ' identical ' twins are never 

 absolutely alike, and the members of a litter of dogs, pigs, or 

 kittens often differ greatly in size, shape, colour, strength, activity, 

 mental disposition, length and texture of hair, and every other 

 character. These differences cannot be due to the direct action 

 of the environment, for the germ-cells, embryos, and foetuses 

 existed from first to last under conditions that were practically 

 identical. They must, therefore, be spontaneous. At any rate, I, 

 for one, cannot believe that such small differences of environment 

 as may have existed before birth can result in one puppy being 

 big and black and another small and brown, in one being rough 

 and the other smooth-haired, or in one resembling the father, 

 another the mother, and a third a remote ancestor, and so on. 

 Moreover, it has been observed that, when the parents are the 

 same, differences between members of the same litter are just as 

 great as those between members of successive litters. It follows, 

 then, that some variations at least are spontaneous, and our 

 task in the present chapter is to ascertain, not whether ALL variations 

 are due to the direct action of the environment, but whether AN\ 

 variations are due to it. 



129. With the intention of proving that variations are due to 

 the direct action of the environment a vast amount of evidence has 

 been accumulated by biologists. A little of it is fairly conclusive, 

 much is inconclusive, and much has no bearing on the point 

 at issue. 



130. Zymotic diseases malaria, tuberculosis, typhoid, dysentery, 

 cholera, diphtheria, and the like are produced by different species 

 of unicellular organisms. Bacteriologists are able to increase or 

 decrease the virulence of many diseases by altering artificially the 

 environment of the microbes. Thus small-pox, when removed to 



1 See 163. 



