7()8 LIFE : OUTLINES OF GENERAL BIOLOGY 



towards gradual formation of distinct species; and he practically 

 restricts the evolutionary importance of natural selection to that 

 of mutations alone. The cause of such large variations remains 

 unexplained; as for the smaller ones mainly relied on by Darwin, 

 they remain "spontaneous and indefinite" accordingly, pending 

 interpretation. 



De Vries assumes — as seems for any theory reasonable — that 

 variations and mutations alike could and should take place more 

 readily and more frequently in the geologic past; and these the 

 more the further back we go, since the continuity of heredity had 

 then far fewer generations to evolve, in fact to steady itself in. 

 So thus — as early and large mutations — he interprets all the funda- 

 mental characters of great groups, even to the "big hfts", and con- 

 sequent big gaps in continuity, which we have so long been seeking 

 to fill by intermediate forms or missing links; and, despite some 

 fortunate finds, as yet so incompletely. 



L^nfortunately for this Mutation theory, however, its main experi- 

 mental basis — that of the mutations of an American evening prim- 

 rose ((Enothera Lamarckiana) described by him as new species — has 

 been adversely criticised by other and yet more scrupulously careful 

 breeders, who consider these new forms as but reappearances of 

 ancestral characters from what they regard as an uncertainly 

 variable stock, and even more or less complicatedly hybrid as well. 

 So even De Vries himself has since seemed shaken in his convictions. 

 Still, his arguments do not rest on this alone. 



His work has been widely stimulating, and must remain of 

 historic interest as a step and link in progress. For while retaining 

 from Darwin the potent agency of natural selection, and not only 

 reviving, but concentrating on, "sports", to which Darwin naturally 

 gave some importance, De Vries has also been a pioneer of exact 

 genetics. And this not only as one of the three re-discoverers of 

 Mendel's principles in 1900, and helpfully suggestive to later 

 Mendelians, but also, by his "pangene" hypothesis towards inter- 

 pretation of inheritance in its details, to that minute interpretation of 

 the arrangement of genes within the chromosomes of the ovum which 

 T. H. Morgan and so many others have been labouring to de- 

 cipher. Among the supporters of De Vries's general doctrine of 

 mutation, with independent evidence from other plant-types, may 

 be noted Blaringhem, whose Transformations brusques des Etres 

 vivants (Flammarion, 1920) still affords a useful survey of this line 

 of investigation. 



Definitions of Species. — Here we may include a few samples 

 of the many definitions of species that have been proposed: 



"The Species is an ideal entity as much as the genus, the family, 

 the order, the class, or the type (all these equally ideal (or real) in 

 nature). 



