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 prolonged deviation of variability from the mean, of which continued selection of individuals, strongly developed in any particular direction, 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 results of scientific research. Darwin and Wallace were not acquainted with these objections to their theory, it was only long after the publication of their works, that science became aware of the existence 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 faithful 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 explanation. 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-
