February i6, 1899] 



NATURE 



365 



to a right conception of a species ; and this Dr. Dreyer 

 has not given. He gives admirable drawings and de- 

 scriptions of a great number of form-varieties, but he does 

 not tell us how often each variety occurred in the 25,000 

 specimens examined. A table, showing the frequency 

 with which every variety drawn was actually observed, 

 would have added so greatly to the value of the work 

 that it would have been well worth the labour involved in 

 making it. 



The essay is apparently published in the hope of 

 stimulating others to undertake an experimental in- 

 vestigation of some of the factors which determine the 

 shapes of rhizopod shells ; and the author seems to have 

 undertaken his study of Peneroplis as a preparation for 

 experimental work of the same kind. Every reader will 

 wish Dr. Dreyer a full measure of success in the difficult 

 task he has set himself to accomplish. 



For the reason just given, little attempt is made to use 

 the observations recorded as a basis for generalisation ; 

 but the last chapter contains certain criticisms of current 

 biological doctrines, which seem based upon fundamental 

 misconceptions. It is difficult to convey an exact idea of 

 a writer's meaning by quoting short extracts, but the fol- 

 lowing passages express Dr. Dreyer's position fairly 

 well. In discussing the conception "of animal evolution 

 he says : 



" Gesetzt aber audi, die genealogische Entwicklung im 

 ganzen oder in diesem oder jenern ihrer Zweige befande 

 sich voUstandig und sicher in unserem intellektuellen 

 Besitz, so hatten wir hiermit eine Entwicklung, die wir 

 naturgesetzlich eben so wenig verstanden, wie die 

 einzelnen Lebensformen, aus denen sie sich zusam- 

 mensetzt." 



It is, of course, perfectly conceivable that we might 

 know the exact genealogy of all living species, or of some 

 of them, without knowing anything of the process by 

 which the modifications undergone by the ancestral 

 species has been effected ;, but the statement that species 

 are susceptible of modification in the course of genera- 

 tions, if it is true, is itself the statement of a natural law ; 

 so that Dr. Dreyer's meaning is difficult to discover. 



Darwin's hypothesis concerning the process by which 

 specific modification has been effected is dealt with as 

 follows : 



"Nun ist die Selektionslehre einmal falsch, denn sie 

 steht in krassem Widerspruche zur Wahrscheinlichkeits- 

 rechnung, und wenn sie richtig ware, wiirde sie ein 

 Verstiindniss der uns als leibhaftige Probleme entgegen- 

 tretende Organismen im nichts beriihren, ebensowenig 

 wie . . . diese oder jene Aste eines Baumes damit 

 ' erklart ' sein konnen, dass sie der Gartner nicht 

 vveggeschnitten hat. 



" Wenn also die Ergebnisse der Descendenz forschung 

 problematisch sind und, wenn sie sicher wiiren, ober- 

 flachlich, so ist die Selektionslehre in sich hinfallig, und 

 wenn sie richtig ware, ware sie nichtig. 



"Es wird nunmehr Zeit dass die jung aufwachsene 

 Biologie von dieser ihrer englischen Krankheit erhole 

 und mannbar werde." 



It would be interesting to have a more detailed ex- 

 position of the author's reasons for saying that the theory 

 of Natural Selection is in contradiction to the laws of 

 Probability. No case has yet been described in which 

 the phenomena of variation and inheritance have been 

 shown not to obey the law of Probability. If Dr. Dreyer 

 NO. 1529, VOL. 59] 



knows of such a case, he would do well to publish his 

 knowledge. 



The remainder of the criticism is worth serious notice, 

 in spite of the bad taste shown in the last paragraph, 

 because the objection to the use which is often made of 

 the theory of Natural Selection is perfectly just. It 

 is quite true that a plausible hypothesis about the utility 

 of an organ or of a function is not a proof that it has 

 been produced by natural selection ; and when such a 

 phenomenon as Death itself is "explained" by ingenious 

 guess-work of this kind, one feels that much must be 

 forgiven to a hostile critic. But these things are no 

 essential part of the Darwinian theory. Darwin laid 

 down two fundamental propositions— yfrj/, that the 

 differences in structure between individual animals of the 

 same race or species are associated with differences in 

 the death-rate and power of producing offspring, so that 

 the number of descendants left by an individual is a 

 function of its structure ; and, secondly, that the effect of 

 differential fertility, associated with structural differences, 

 is often sufficient to change the character of the race or 

 species in the course of successive generations. 



These are essentially physiological propositions, which 

 admit of direct experimental verification or disproof. 

 The experimental testing of these two propositions would 

 open up a fascinating field of knowledge, which has been 

 left almost untouched since Darwin himself wrote. 



Darwin was so fully occupied in forcing men to recog- 

 nise the broad fact of structural variation, that he had 

 little time to demonstrate the relation between variation 

 and death-rate. Nevertheless, naturalists have been 

 content for forty years to rest a great generalisation on 

 his work alone, without themselves attempting to amplify 

 it by direct observation and experiment. It is time that 

 a systematic study should be made of the relation 

 between structural abnormality and death-rate, under 

 definitely determined conditions of environment, in a 

 large number of species. If the relation postulated by 

 Darwin generally exists (as the writer believes), it is time 

 that it should be properly demonstrated. If it does not 

 exist, it is time that the belief in natural selection should 

 be given up. 



But it is the business of naturalists to formulate the 

 processes of nature as well as they can ; and whether 

 the process of Natural Selection interests Dr. Dreyer or 

 not, we ought to want to know certainly whether it 

 occurs. The statement that such a process does affect 

 animals generally, is either a natural law of great import- 

 ance, or it is untrue. If it is true, it is as absurd to call 

 it " nichtig " because it does not formulate all the pro- 

 cesses of a living organism, as it would be to call it 

 useless because it does not enable one to foretell to- 

 morrow's weather. W. F. R. Weldon. 



OUR BOOK SHELF. 



Elements of Sanitary Engineering. By Mansfield 



Merriman, Professor of Civil Engineering in Lehigh 



University. Pp. 216. (New York and London: 



Chapman and Hall, 1898.) 



The author of this book deals with the whole range 



of sanitary science, including an historical notice of 



sanitation from the time of the Israelites in Egypt ; 



the classification of diseases ; statistics of mortality as 



