484 POPULAR SCIENCE MONTHLY. 



limited to differences in the degree of development of those already 

 extant ? Most of these questions were left unanswered at the time and, 

 for the greater part, have remained so. And, as long as no answer 

 was forthcoming, imagination had free play as regards the manner in 

 which one species originated from another. 



A stop was put to this when Quetelet discovered his famous law. 

 Variability obeys certain rules; nothing outside the compass of these 

 rules can be attributed to it. Variability is not unlimited and always 

 returns to its starting point. There may be various causes for a pro- 

 longed deviation of variability from the mean, of which continued 

 selection of individuals, strongly developed in any particular direc- 

 tion, is the most important; but as soon as these causes cease to exist 

 or this selection ceases to be practiced, it must return to the mean. 

 Variability is nothing but a more or less, a plus-variation or a minus- 

 variation; it does not go in any direction other than the greater or 

 lesser development of a character already present. Variability merely 

 causes a decrease or an increase; it does not create. 



It remained for the disciples of Quetelet to draw attention to the 

 consequences of his discovery, which are among the most recent re- 

 sults of scientific research. Darwin and Wallace were not ac- 

 quainted with these objections to their theory, it was only long after 

 the publication of their works, that science became aware of the exist- 

 ence of these objections and of their importance. 



The theory of variability, such as we know it at the present day, 

 does not lead to conclusions favorable to the theory of the gradual 

 origin of species, the theory which assumes that species originated by 

 a gradual increase in the degree of variability. Hence many writers 

 have at various times declared more or less openly against this theory. 

 Others again have tried to reconcile it with the newly discovered facts. 

 But Darwin's explanation is a most plausible one, which, apparently 

 at least, solves all difficulties. And the voice of his antagonists is as 

 yet not so powerful but that the great majority should remain faith- 

 ful to the old banner. 



Besides, Darwin never expressed himself so definitely upon this 

 point as some would have us believe. Openly in one passage, less so 

 in a second one, he acknowledges the possibility of another explana- 

 tion. It might very well be possible that the changes of the species 

 in nature might occur suddenly, as had been observed to be the case 

 in agriculture and horticulture. This would, as satisfactorily as the 

 theory of gradual change, explain the relationship existing between 

 smaller species in nature and more especially between those agricultural 

 plants which systematic botany unites into a single species. Without 

 a doubt, the formation of the various kinds of beets, of oats or of 

 barley, would have required many centuries, but the results are ac- 



