668 



SCIENCE. 



[N. S. Vol. XXII. No. 569. 



While so much is claimed * * * as due to 

 climatic causes, it is admitted also that habits 

 and food, and other conditions of life than those 

 resulting from climate, have a marked effect in 

 determining modifications of form and color 

 among animals. A scarcity of a favorite kind 

 of food Vi^ill undoubtedly force species to subsist 

 upon the next best that offers, which may be so 

 different as to modify certain characters and fit 

 the species to live upon the less desired food. 

 A change of food may lead to modification of 

 dentition, the muscles of mastication, and the 

 organs of digestion, and, eorrelatively of other 

 organs or parts of the body; the modification, 

 however, arising simultaneously among all the 

 descendants of the individuals thus driven to a 

 change of diet, instead of appearing first in a 

 single individual and becoming perpetuated in its 

 descendants alone. Entomologists have found 

 that, among insects of the same species, the forced 

 or voluntary use of different food-plants gives rise 

 to modifications of color and structure, and hence 

 results in what have been termed phytophagic 

 , varieties or subspecies, and that man can also 

 affect such changes at will by simj^ly changing the 

 food of the species. Again, the geological char- 

 acter of a country is well known to have a 

 marked effect upon the size and color of animals 

 inhabiting it, as is strikingly illustrated among 

 molluscous animals, whose abundance, and even 

 presence, is largely dependent upon the constitu- 

 ents of the soil. Over regions of the United 

 States, for example, where the underlying rock 

 is non-calcareous, the species are both few in 

 number and sparsely represented. In respect to 

 the fresh-water mussels, those of the same species 

 from different streams are easily distinguishable 

 by differences in the thickness of the shell, in 

 color, shape, and ornamentation, so that the char- 

 acter of the shells themselves affords a clew to 

 the locality of their origin. At some localities 

 the species tend to become tuberculous or spinous 

 * * *; at other localities, they acquire a very 

 much thickened shell, or different colors, the 

 same characteristics appearing simultaneously in 

 quite diverse species, and thus becoming distinctive 

 of particular localities. [After reference to mam- 

 mals of certain regions being influenced in rela- 

 tion to size by the presence of calcareous or non- 

 calcareous soils, and to the birds of the Galapagos 

 Archipelago, with their short wings and large 

 bills, etc., there follows:] The sedentary life 

 necessitated by the confined habitats of species 

 thus situated would naturally act more or less 

 strongly on the organs of fiight, and a reduc- 



tion in the size of the wing would follow; not 

 necessarily through the round-about process of 

 natural selection, through the modification orig- 

 inally of a single individual, but by the direct 

 action on all the individuals alike of the changed 

 conditions of life. 



There are thus what may be termed re- 

 gional modifications due to the direct ac- 

 tion of environment, independently of 

 natural selection, in its original, restricted 

 sense, or of isolation. The modifying in- 

 fluence may be either primarily climatic 

 or due to peculiar constituents of the water 

 or soil and the resultant vegetation. In a 

 sense the two latter conditions may act as 

 barriers, with the resultant effect of modi- 

 fied isolation. In general, however, in 

 birds and mammals, in which regional 

 modifications are so patent, the main fac- 

 tor is climate, the action general, and the 

 transition between regions gradual. "While 

 all these influences may be as active on 

 islands as on continents, there is in the 

 former the powerful agency of isolation 

 superimposed upon all the other agents 

 that tend to the differentiation of animal 

 forms. J. A. Allen. 



SCIENTIFIC BOOKS. 

 The Evolution Theory. By Dr. August Weis- 

 MANN, Professor of Zoology in the Univer- 

 sity of Freiberg in Breisgau. Translated 

 with the author's cooperation by J. Arthur 

 Thomson, Regius Professor of Natural His- 

 tory in the University of Aberdeen, and 

 Margaret R. Thomson. London, Edward 

 Arnold. 1904. 2 vols., illustrated. Pp. 

 416 and 405. 



No one, in the last thirty years, has con- 

 tributed more to the discussion and investiga- 

 tion of evolutionary problems than has August 

 Weismann. The present work marks the cul- 

 mination of his long series of stimulating 

 writings. His fertility in hypotheses and 

 keenness of criticism are well known; not less 

 noteworthy is his readiness to withdraw hy- 

 potheses when disproved, or to modify them 

 to conform with new discoveries. Thus, the 



