CH. XI] APPLICATION OF AGE AND AREA m 



the distribution of species about the globe, have been chiefly 

 determined by age, when one is deahng with the mass of species 

 the various other causes that may be operative— cUmatic eco- 

 logical, geographical, geological, etc.-simply causing deviations 

 to one side or the other, but not permanently divertina the 

 dominant plan. Age and Area obviously, therefore, becomes a 

 corollary of the larger law. 



But if this be so universal a rule in plants, it is obvious that it 

 must probably show in animals also, and Chapter xix shows 

 that this IS actually the case, and that it is exhibited as clearly 

 in the animal kingdom as in the vegetable. 



The question of Origin of Species is 'then touched upon 

 (Chapter xx), and it is shown that probability is much in favour 

 of mutation as against infinitesimal variation, and that the 

 effect of the recent work upon distribution and evolution de- 

 scribed in this book is to make extremely probable the con- 

 tention that I have frequently put forward, and which is now 

 accepted by Prof, de Vries, that mutations may at times occur 

 of the necessary "size" to give rise at once to Linnean species 

 If one such mutation sur^•ived in fifty years, the whole existing 

 population of flowering plants could be evolved in eight million 

 years, which is perhaps less than 25 per cent, of the time that 

 has actually been available for, and occupied in, their evolution. 

 If c^'olution be a predetermined result, then it is clear that 

 advantage as guiding it is ruled out of acceptance, and it is 

 difficult to see, upon this ground alone (though there is strong 

 evidence upon other grounds), how anything but direct mutation 

 giving Linnean species can be effective. 



In the following chapter (xxi) Prof, de Vries deals with the 

 relations of Age and Area to the Mutation theory, first ])ointing 

 out the essential diffe-ence between this and the theory of 

 infinitesimal variation. In the latter there is no change in the 

 genes, or material bearers of characters, but merely a fluctuation 

 or oscillation of the emphasis of the characters about a mean 

 value, so that in one member of a group of plants of common 

 descent a character may be large, in another small, and so on. 

 In the theory of Mutation, the changes have invoh'ed the genes, 

 the alterations in these resulting in permanent and usually 

 hereditary differences in the organism. 



Prof, de Vries then points out that while Darwin recognised 

 that both mutation and fluctuation might result in new species, 

 the material of facts at hand was insufficient for any kind of 



