EVOLUTION AND MUTATION 5 



doubt that it proves that the current view of extremely slow 

 and almost invisible changes must be abandoned. 



Shortly after the publication of Darwin's Origin ofSpecies, 

 the Belgian anthropologist, Quetelet, submitted the variabil- 

 ity in measurement of the different parts of the human body 

 to a statistical investigation. He discovered that this kind 

 of variabihty follows distinct laws and that these laws agree, 

 in the main, with the law of probabiUty. Small divergences 

 from the average are numerous, larger discrepancies are 

 rare, and the rarer, the larger they are. Variabihty is thereby 

 limited, and is subject to a return to the average condition. 

 It may be moved from tliis average, to some extent, by a 

 change in the outward conditions or by a repeated selection 

 in one direction ; but, as soon as these causes and this selection 

 cease to work, a return to the average is unavoidable. Vari- 

 abihty may augment or diminish the quahties; it is linear, 

 consisting of changes along a simple hne, some being positive 

 and others being negative, but it does not strike into new 

 directions. It is no source of new r^uahties. The phenom- 

 ena which are controlled by this law and which are bound 

 to such narrow limits cannot be a basis for the explanation 

 of the origin of species. It governs cjuantitics and degrees 

 of cjuahties, but not the quahties themselves. Species, 

 however, are not, in the main, distinguished from their 

 alUes by c^uantities or by degrees their very quahties may 

 differ. 



From this discussion it may be seen that the slow and 

 gradual changes of ordinary variabihty and the production 

 of new characters are not of the same order. Variabihty, 

 in the ordinary sense of the word, is a broad conception. 

 It must be subdivided for the purpose of scientific investiga- 

 tion. The phenomena that follow Quetelet's law are now 

 considered as one group, which is called fluctuating variabihty 

 or fluctuation, since the individual quahties fluctuate around 



