598 



SCIENCE. 



[N. S. Vol. XX. No. 508. 



to reversion to mediocrity, always more or 

 less returning in the progeny to the pre- 

 vious state. Species, on the contrary, are 

 usually constant and do not commonly or 

 readily revert into one another. It is as- 

 sumed that from time to time specific re- 

 versions occur, but they are too rare to be 

 comparable with the phenomena which are 

 ruled by the law of probability. 



A thorough study of Quetelet's law would 

 no doubt at once have revealed the weak 

 point in Darwin 's conception of the process 

 of evolution. But it was published as part 

 of a larger inquiry in the department of 

 anthropology, and for years and years it 

 has been prominent in that science, with- 

 out, however, being applied to the corre- 

 sponding phenomena of the life of animals 

 and of plants. Only of late has it freed 

 itself from its bounds, transgressed the old 

 narrow limits, and displayed its prominent 

 and universal importance as one of the 

 fundamental laws of living nature. 



In doing so, however, it has become the 

 starting point for a critical review of the 

 very basis of Darwin's conception of the 

 part played by natural selection. It at 

 once became clear that the phenomena 

 which are ruled by this law, and which are 

 bound to such narrow limits, can not be a 

 basis for the explanation of the origin of 

 species. It rules quantities and degrees of 

 qualities, but not the qualities themselves. 



Species, however, are not in the main 

 distinguished from their allies by quanti- 

 ties, nor by degrees ; the very qualities may 

 differ. The higher aiiimals and plants are 

 not only taller and heavier than their long- 

 forgotten unicellular forefathers ; they sur- 

 pass them in large numbers of special char- 

 acters, which must have been acquired by 

 their ancestors in the lapse of time. How 

 such characters have been brought about is 

 the real question with which the theory of 

 evolution is concerned. Now if they can 

 not be explained by the slow and gradual 



accumulation of individual variations, evi- 

 dently the second alternative of Darwin's 

 original proposition I'emains. This was 

 based on the sports, on those rare and sud- 

 den changes which from time to time are 

 seen to occur amongst cultivated plants, 

 and which in these cases give rise to new 

 strains. If such strains can be proved to 

 offer a better analogy to real systematic 

 species, and if the sudden changes can be 

 shown to occur in nature as well as they 

 are known to occur in the cultivated con- 

 dition, then in truth Darwinism can afford 

 to lose the individual variations as a basis. 

 Then there will be two vast dominions of 

 variability, sharply limited, and sharply 

 contrasted with one another. One of them 

 will be ruled by Quetelet's law of prob- 

 ability, and by the unavoidable and con- 

 tinuous occurrence of reversions. It will 

 reign supreme in the sciences of anthro- 

 pology and sociology. Outside of these, the 

 other will become a new domain of investi- 

 gation, and will ask to be designated by a 

 new name. Fortunately, however, a real 

 new designation is not required, since pre- 

 vious to Darwin's writings the same ques- 

 tions were largely discussed, and since in 

 these discussions a distinct name for the 

 sudden and accidental changes of species 

 into one another was regularly used. At 

 that time they were called ' iputations, ' and 

 the phenomenon of mutability was more or 

 less clearly distinguished from that of va- 

 riability in a more limited sense. Espe- 

 cially in France a serious scientific conflict 

 raged on this point about the middle of the 

 last century, and its near relation to re- 

 ligious questions secured it a large interest. 

 Jordan and Godron were the leaders and 

 numerous distinguished botanists and zool- 

 ogists enrolled themselves under their ban- 

 ners. They cleared part of the way for 

 Darwin and collected a large amount of 

 valuable evidence. Their facts pleaded for 

 the sharp and abrupt delimitation of their 



