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SCIENCE 



[N. S. Vol. XXVI. No. 676 



of late regarding dominance, recessiveness, 

 latency, etc., and it is highly desirable to keep 

 in mind the fact that all these abstractions 

 are really expressions of our ignorance. 



Although the terms " character " and " unit- 

 character " are frequently employed no defi- 

 nitions are given nor do we find any discus- 

 sion of the possible nature of characters ex- 

 cept in the section on sex determination, where 

 morphological and physiological conceptions 

 are briefly contrasted (pp. 420-422), the au- 

 thor favoring the physiological. 



As in earlier writings, Morgan follows 

 de Vries in drawing a sharp distinction be- 

 tween mutations and fluctuating variations 

 and inclines toward the view that species arise 

 by mutation and that selection does not orig- 

 inate but merely eliminates. 



The second section, " Experimental Study 

 of Growth," is rather brief, comprising an 

 introductory chapter, in which normal growth, 

 senescence, length of life, and absorption of 

 parts are briefly discussed; a second chapter, 

 on " External Factors that Influence Growth," 

 and a third, on " Growth and Regeneration." 

 Any treatment of the subject of growth on 

 an experimental basis from which the data of 

 experimental embryology and form-regulation 

 are excluded must of necessity be incomplete 

 and in certain respects unsatisfactory, and 

 this section does not escape these disadvan- 

 tages of limitation. 



In the introductory chapter growth is de- 

 fined as " an increase in the volume of the 

 living material" (p. 240). According to this 

 definition the increase in volume of bone, 

 shell and other skeletal structures is not 

 growth. Morgan himself is not consistent in 

 his use of the term; within half a page of the 

 definition he speaks of " steady and rapid 

 growth due to imbibition of water " (p. 241) 

 and on page 258, in discussing the effects of 

 salts on growth, the experiments of Herbst and 

 Maas on the relation between certain salts and 

 the development of the skeleton in sea-urchin 

 larvae and sponges are cited. 



Moreover, since it is expressly stated in 

 connection with the definition that change of 

 form is not necessarily associated with growth, 

 this section, according to the author's position, 



has no connection with the main theme of the 

 book, " the determination of the conditions 

 under which changes of form occur " (p. v, 

 preface). 



The chapter on " Growth and Regenera- 

 tion " is not, as its title might lead one to ex- 

 pect, a consideration of the relation between 

 normal and regulatory growth, but a discus- 

 sion of certain experiments on the rate of 

 regeneration at different levels and the au- 

 thor's pressure-tension hypothesis. Since this 

 is the fullest statement of this hypothesis that 

 Morgan has presented, portions of it are given 

 here in his own words: 



What factor determines that the terminal or- 

 gans are those that are first laid down in the 

 new part? ... A number of considerations, that 

 I can not enter into more fully here, have led me 

 to suspect that this relation of the parts can be 

 accounted for as due to a condition of stratifica- 

 tion or polarity, due to the mutual pressure of 

 the parts on each other, which acts as the stim- 

 ulus for the differentiation of the cells. By these 

 same assumptions we can, I think, also give a 

 fairly consistent explanation of the difference in 

 the rule of growth at different levels (p. 280). 



And again in discussing the formation of 

 head and posterior end from anterior and 

 posterior cut surfaces in Lumbri cuius he says : 



Since the development of these new parts seems 

 to be largely a centripetal phenomenon, we can 

 not assume that the influence of the old part on 

 the new, a centrifugal influence, determines the 

 result; but since the order or sequence of the 

 differentiation in the new part is the same as that 

 in the old part this may determine whether a 

 head or a tail develops. . . . The centripetal in- 

 fluence acting on the new material at the anterioi 

 end determines therefore that this is a head, and 

 acting on the new material at the posterior end 

 determines that this is a tail. The centripetal 

 influence is, according to my interpretation, noth- 

 ing more than the tension of the outer layer of 

 cells and the pressure relations in general, in the 

 rounded dome-shaped mass of new materials. In 

 this way we can give a formal solution of the 

 development of a head in one case and of a tail 

 in the other. 



Let us see whether the same hypothesis will 

 explain the different rates of growth of the pos- 

 terior end according to the level of the cut, as 

 seen in the earthworm, salamander and fish. A 

 growing region is present near, but not quite at, 



