VARIATION. ADAPTATION. ORIGIN OF SPECIES 395 



alteration in the mutating organism. The new characters must, as Nageli 

 (1884) acutely pointed out, be firmly fixed and ready as initials which are at first 

 latent, and later on unfold themselves. When an initial once finds the way to 

 complete development then it may go on elaborating in ever-increasing degree. 

 In this way also certain new characters would behave just Uke certain ancestral 

 characters which make their appearance as so-called ' reversions ' whenever op- 

 portunity offers, and which generally are transmitted in the latent form. We 

 know as little about the causes which lead to their appearance as we do of the 

 origin of mutations. All we can say is that the causes appear to be internal, 

 which is equivalent to saying that we know nothing about thern. 



If then, as De Vries thinks, the mutation, and not the individual variation, 

 is the factor concerned in the origin of new species, then selection must have 

 a significance quite different from that attributed to it by Darwin. According 

 to Darwin, individuals are for ever engaged in a struggle for existence, and new 

 species arise by selection of the varieties best equipped for the struggle. Accord- 

 ing to De Vries, however, fully developed species come into conflict, but the 

 origin of these species by mutation is not thereby explained. We have certainly, 

 if we accept De Vries' s view, a ' Mutationstheorie ' but we have absolutely no 

 theory of mutation itself. 



The formation of species farther apart out of the fundamental species is, 

 according to De Vries, easily understood, since 

 many 'petites especes' disappear in the struggle 

 for existence. Of course, the mutation may be 

 so great that a new genus or a new family 

 may come into existence. Capsella heegeri, 

 for example, would scarcely have been placed 

 in the genus Capsella were it not that its origin 

 was known. It is also quite possible that whole 

 genera and families may be referred back in their ^.^ ^^3 ,;, „,,.,,, ^, 



chief features to monstrosities. HiLDEBRAND Fuchsia. /, lateral view. //, ground 



(1899) has found Fuchsia to produce zygomor- p'^"- After hildebrand(i899). 

 phic flowers arising by mutation (Fig. 118), so 



that the allied genus Lopezia may have arisen from a malformation (Compare 

 Sachs, 1893). 



Hofmeister long ago (1868, p. 564) ascribed to mutation a very prominent 

 part in the formation of species. He said : ' New forms do not come into 

 existence by the summation in successive generations of small differences from 

 the customary form, all tending in the same direction ; they appear suddenly, 

 and are widely different from the parent type.' It may well be that, as our know- 

 ledge increases, the distinction between the different types of variation may be 

 broken down ; still it is certain that mutability will remain the chief if not the 

 only factor in species formation. That species may also owe their origin to 

 individual variation, in the extreme way suggested by Wallace, appears to us 

 practically out of the question, but that they may rise by hybridization is, in 

 individual cases, quite established, and that they may owe their origin to adapta- 

 tion is especially true of those species (biological species) recently recognized 

 among parasitic Fungi. Unfortunately, space will not permit of our discussing 

 these forms. [Compare Klebahn, 1904, and Detto, 1904.] 



In this lecture we have not been able to give more than the briefest sketch 

 of the theory of the origin of species, and have been compelled to omit considera- 

 tion of very many most important observations and views. We must, therefore, 

 refer to the special literature on the subject, in which, it is true, some one ' theory ' 

 frequently predominates. The task of the future is less to establish new theories 

 than carry out systematic observations and experiments. General summaries 

 of the Theory of Descent will be found in the following works : — Wallace, 1891 ; 



