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THE AMERICAN NATURALIST [Vol. XLVII 



the two fields mentioned. Lloyd's distinctive contribution is the 

 demonstration that there exist in certain wild organisms the 

 same conditions that are found in experimentation. 



The author's interest lies mainly in bringing the work into 

 relation with the species concept as employed in systematic zool- 

 ogy, and in showing its opposition to what he calls the " accepted" 

 view that diverse groups of organisms arise gradually, by a proc- 

 ess of selection among minute gradations. He shows that many 

 of the so-called species of rats are distinguished from each other 

 in the same way as are the small groups he has observed ; there- 

 fore there is not ground for supposing all the members of such 

 species to be descended from a common stock. "Animal species 

 appear to be conventional rather than real" (p. 117). But 

 among the author's fluctuating views on this point is the doubt- 

 ful assertion that "there seems to be reason for believing that the 

 distinction between specific and varietal characters which was 

 recognized by De Vries in plants is also recognizable among 

 higher animals" (p. 139) ; which would seem to imply that there 

 is some sort of reality underlying the distinction of species. The 

 matter is not analyzed with precision. 



In a special chapter the author attempts to apply to certain 

 problems of disease and abnormality in man the idea that new 

 forms arise by mutation. Here again, those who have at hand 

 such a work as Davenport's "Eugenics" will find the presenta- 

 tion vague, though tending in the same direction as in the work 

 mentioned. 



In a concluding chapter the author abandons empirical evi- 

 dence for an attempt to criticize the theory of natural selection 

 from a general and philosophical point of view. Much is made 

 of the supposed requirement that variation should be "in all 

 directions," the criticism depending "on the assumption that we 

 can not imagine things varying in all directions" (p. 175). 

 Further, adaptation is held not to be a phenomenon needing 

 explanation. The rest of the discussion is of a similar character, 

 showing less precision of analysis than any other part of the 



The author states in the preface that the book is offered "as an 

 assortment of opinions which may be of suggestive value." The 

 observed facts in regard to the diverse groups of rats, however, 

 make it more than this. 



H. S. Jennings 



