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THE POPULAR SCIENCE MONTHLY. 



quate to account for the amount and intensi- 

 ty of the energy. There are cited as sup- 

 porting the impact theory, or as illustrating 

 it, the meteorites, which may be residual por- 

 tions of some of the original solid bodies; 

 comets, for which a similar origin may be 

 supposed ; the motions of the stars, which 

 are of greater velocity than can result from 

 gravitation ; the facility with which the 

 theory will explain nearly every feature of 

 the nebulae; and binary stars, sudden out- 

 bursts of stars, and star clusters. An argu- 

 ment is based on the insufficiency of the 

 gravitation theory to account for the heating 

 of the primary nebula, while the " impact 

 theory " furnishes at once a sufficient origin 

 for it ; and another, which is styled " a cru- 

 cial test," on the requisitions of geological 

 time as dependent on the antiquity of the 

 sun's heat. It is mathematically demon- 

 strable that, if gravitation be the only source 

 from which the sun derived its heat, life on 

 the globe can not date further back than 

 twenty million years ; and attempts have 

 been maJc to measure the geological ages by 

 this rule. Mr. Croll argues, from the evi- 

 dences afforded by the amount of denudation 

 that has occurred, and its calculated rate, 

 and by biological development, that the pro- 

 cesses which have taken place can not be 

 subjected to such limitations. Further light 

 is cast upon the theory by citations from the 

 views, or consideration of questions suggested 

 by them, of Prof. A. Winchell, Mr. Charles 

 Morris, Sir William R. Grove, Sir Benjamin 

 Brodie, Dr. T. Sterry Hunt, Mr. William 

 Crookes, Prof. F. W. Clarke, and Dr. G. 

 Johnstone Stoney, on the prenebular condi- 

 tion of matter. 



Darwinism : An Explanation of the Theory 

 of Natural Selection, with some of its 

 Applications. By Alfred Russel Wal- 

 lace. London and New York : Macmillan 

 & Co. Pp. 494. Price, $1.75. 



This work treats of the origin of species 

 on the same general lines as were adopted 

 by Darwin, but in the light of the discus- 

 sions, objections, theories, and new discov- 

 eries that have been brought forth in the 

 nearly thirty years which have elapsed since 

 Darwin promulgated his great principle. 

 The objections made to Darwin's theory in its 

 earlier days were fundamental, and were di- 

 rected against the principle itself. But Dar- 



win " did his work so well that ' descent with 

 modifications ' is now universally accepted as 

 the order of Nature in the organic world; 

 and the rising generation of naturalists can 

 hardly realize the novelty of this idea, or 

 that their fathers considered it a scientific 

 heresy to be condemned rather than serious- 

 ly discussed." The objections now made to 

 the theory apply solely to the particular 

 means by which the change of species has 

 been brought about. The objectors seek to 

 minimize the agency of natural selection, and 

 to subordinate it to laws of variation, of use 

 and disuse, of intelligence and heredity. Mr. 

 Wallace maintains the overwhelming impor- 

 tance of natural selection over all other 

 agencies in the production of new species. 

 He begins with illustrating the struggle for 

 existence, which he considers one of the 

 most important and universal, and yet least 

 understood, forces of Nature. Next, varia- 

 bility is shown to be constant, universal, in- 

 cessant, and frequent. It was a weakness in 

 Mr. Darwin's argument that he based it so 

 largely on the evidence of domesticated ani- 

 mals and plants. Mr. Wallace goes to Nature, 

 and finds variation just as much the rule 

 with species in the wild state, illustrating the 

 fact with numerous citations and diagrams ; 

 and the objection that the preponderance of 

 chances is immensely against the right varia- 

 tion or combination of variations occurring 

 just when required, is blown away by show- 

 ing that all .forms of variation are all the 

 time occurring. The argument is continued 

 as to the relations of crosses, color, mimicry, 

 heredity, and the geographical distribution 

 of organisms. The objection based upon the 

 failure to find evidences of the existence or » 

 former existence of a great number of the 

 connecting links, which the theory of evolu- 

 tion supposes must have been developed, is 

 answered by showing that the geological 

 record of former forms is, and always will 

 be, very imperfect, particularly with reference 

 to animals and plants of the upland; and 

 good reasons are given to show why it must 

 be so. The views of Mr. Spencer, as set 

 forth in his " Factors of Organic Evolution," 

 and of Prof. Cope, Dr. Karl Semper, Prof. 

 Geddes, and Prof. Weismann, are taken up, 

 and claimed not materially to diminish the 

 importance of natural selection, or to show 

 that any of the laws or forces to which they 



