BOOK KEV1EWS. 



415 



What causes them to appear ? Eeferring to poultry the authors say that 

 " faults " occur so frequently in each breed, that the fact shows how 

 strong is the tendency to vary in certain definite directions (64). Apart 

 from " reversions," the authors can only suggest as a cause [?] that there 

 is evidence to show that variations frequently occur along certain definite 

 lines only (68). 



With regard to mutations, they observe of De Vries' experiments 

 with plants, " We believe that some of his conclusions are applicable to 

 plants. We are far from accepting his theory of mutations in toto ; we 

 are, however, convinced that he is on the right track" (75, 76). It is 

 described thus : " First arises a well-marked variety, by a single mutation. 

 Subsequent mutations follow, so that a distinct race is produced. And, 

 finally, fresh mutations occur, so that a new species is eventually pro- 

 duced " (88,89). The authors' view is thus expressed: "We think it 

 probable that all species throw off, at greater or less intervals, discon- 

 tinuous variations, and that it is upon these that natural selection acts " 

 (106). 



Thus far the authors do not appear to differ from Darwin. The 

 latter says, in fact, " Given indefinite variations, natural selection deter- 

 mines the fittest to survive." The authors would say : " Given muta- 

 tions," &c. But mutations are only variations after all ; and if it be asked 

 why they differ from the parent so as to be called mutations, the answer 

 brings us to the ultimate cause — recognized by Weismann and Darwin — 

 " the changed conditions of life." The effect, as seen in the " response," 

 may be small or considerable, at once, or after accumulative influences 

 have taken place ; and it then appears as a "sport" or mutation, which 

 perhaps may be hereditary, or not at all, for reasons unknown to man. 

 Next, as to correlations, we would interpret these not as being the 

 result or effect of one changing organ upon all the rest of an organism ; 

 but the whole being responds simultaneously in an adaptive direction ; or 

 may be in an "injurious " direction under domestication, as when a bird 

 " sports " in producing a tuft of feathers on the head, the skull is so 

 affected by "correlation" that it is far too thin to resist the slightest 

 blow (Tegetmeier). 



With regard to mimicry, we think the authors give the true explana- 

 tion. " The true interpretation of the resemblance is probably that both 

 squirrels and tupaias are adapted to a life in trees. Like profession 

 begets like appearance ; the ground-living shrews much resemble mice, 

 and the moles find representatives in mole-like rodents " (180). Still 

 better illustrations are in the various members of the Marsupials mimick- 

 ing various types of mammalia, &c. Their similar "characters" were, 

 therefore, "acquired" or "begotten," as the authors say, and are now 

 hereditary. 



The authors describe four kinds of mutations in animals (339 ff.), and 

 express their own view thus : " Evolution proceeds by mutations, which 

 may be large or small " (344). 



The eighth and last chapter deals with the " Factors of Evolution." 

 The authors say, " The first of these factors considered, is the tendency 

 of organisms to vary along definite lines " (345). Darwin did not recog- 

 nize any inherent "tendency" to vary, apart from the direct action of 



