Referate. 201 



book is rather to discuss the wider, chiefly unsolved, problems of genetics 

 than to present facts which are already known, and no reader of it can 

 get even to the end of the introductory chapter without admitting, what 

 is often insufficiently realized, that great as are the advances in our 

 knowledge of genetics made in recent years, they carry us only a short 

 way towards understanding the fundamental problems of evolution. There 

 is a tendency at present, especially among those engaged in Mendelian 

 research, to assume that the key has been found to all tlie questions which 

 before 1900 appeared so perplexing, and it is refreshing to read, from the 

 pen of one who has done more than any other to place the study of 

 Mendelian inheritance on a firm basis, that "as to almost all the essential 

 features, whether of cause or mode, by which specific diversity has become 

 what we perceive it to be, we have to confess an ignorance nearly total" 

 (p. 24S); and "when the work [of investigating genetic behaviour] has been 

 done on a scale so large as to provide generalisations, we may be in a 

 position to declare whether specific difference is or is not a physiological 

 reality'^ (p 250). To some this critical attitude may appear discouraging, 

 but when the reader finds that almost every page of the book is illu- 

 strated with a wealth of examples, most not very generally known, and 

 that the problems are treated without personal bias, even when the 

 criticism is destructive, he will probably begin to feel that he is being 

 introduced into regions which are outside the ordinary scope of recent 

 writing on genetics, and that there is no danger of the exhaustion, in the 

 near future, of the Undiscovered. 



The first chapter introduces the problem of specific diversity; it shows 

 that the belief in the fixity of species only arose with Linnaeus, although 

 his acceptance of it is doubtful, and then proceeds to point ont that, 

 contrary to the expectation of Darwin, systematists still adhere to the 

 conception of specific definiteness. They do so, because in fact most species 

 are definite, even in minute points, many example of which are given. But 

 when several species, so differing, hve together under identical conditions, 

 the question arises, why do they differ? We cannot find that one is better 

 suited to its environment than another, or that their differences are in any 

 way adaptive, and we have to conclude "that tolerance has as much to do 

 with diversity of species as the stringency of Selection". Again, some 

 species are very constant; nearly allied species very variable, yet each seems 

 equally well adapted to its surroundings. We are thus driven to suppose 

 that variation and stability are rather an index of internal constitution 

 than of the relations with the outer world. 



The second and third chapters deal with meristic variation, and the 

 phenomena of division. The difference between orderly and disorderly 

 arrangement of parts is emphasized; examples are given of each and it is 

 suggested that "germ-cells differ from somatic cells in the fact that their 

 differentiations are outside the geometrical order which governs the differ- 

 entiation of somatic cells". Under the heading of segmentation and sym- 

 metrical division the metaphor of waves is introduced, without, however, 

 indicating to what kind of force the waves are due. 



In the chapters on Substantive Variation and Mutation there is less 

 which is outside current ideas on these subjects, but in the succeeding 

 chapters on Variation in relation to locality and climate it is again manifest 

 how inadequate is our present knowledge for deahng with the problems. 

 Many examples are given, from both animals and plants, showing how in 

 one case a species may be uniform in one area, uniform but distinct in 



