OTHER FACTORS OF EVOLUTION 135 



this force is cumulative in successive generations. All the observed 

 facts in the experience with herds and flocks point in this 

 direction." • 



Apart, however, from the question of adaptabiUty we must not 

 lose sight of the remarkable changes recorded in the last chapter 

 in illustration of ' mutation.' These changes would, of course, be 

 regarded by Weismann as due to what he calls 'spontaneous' 

 changes in the germ-plasm. But no distinct boundary-line can be 

 drawn between cases of ' induced ' changes in the germ-plasm and 

 those which he speaks of as being ' spontaneous.' Has he not 

 himself said that all such changes " have their roots in external 

 influences" ? And in the case of the tall maize, in the various 

 new species of the Evening Primrose, as well as in that of the 

 black-shouldered Peacock, we have notable instances of acquired 

 characters being transmitted to succeeding generations. 



It is, however, changes strictly belonging to Weismann's cate- 

 gory of ' induced ' germinal selection which will be found to afford 

 the most weighty evidence for the solution of our problem, and in 

 regard to these cases the statements made by Weismann are most 

 emphatic and precise. He says, for instance, " It is indubitable 

 that external influences, such as those emanating from the environ- 

 ment or media in which species live, are able to cause direct variation 

 in the germ-plasm, that is, permanent, because hereditary variations. 

 We have already referred to this process and called it ' induced ' 

 germinal selection " (II, p. 267). While again he speaks of the 

 germ-plasm as " ready and able to furnish any variation that is 

 possible in a species, if that is required by external circumstances " 

 (II, p. 196). 



Here the real point in dispute seems to be frankly admitted. 

 And the fact that acquired characters can be inherited is the real 

 point of importance : it is, as I have said, altogether a matter of 

 subordinate interest whether the change in external conditions acts 

 first upon the soma, or first upon the germ-plasm, or simultaneously 

 upon both. 



We have, however, other evidence bearing upon this latter 

 point, which must not be lost sight of. Thus, that heritable 

 variations may be created in, and transmitted by, the soma only 

 seems to be shown by the phenomena of pelorism, to which 

 reference was made in the last chapter (p. 106), and of bud- 



' Quoted by Cope in " The Primary Factors of Organic Evolution," p. 424. 



