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 of Species, 

 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 variability follows distinct laws and that these laws agree, 

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

 from the average are numerous, larger discrepancies are. 

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

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

 It may be moved from this 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 \vork, a return to the average is unavoidable. Vari- 

 ability may augment or diminish the qualities; it is linear, 

 consisting of changes along a simple line, some being positive 

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

 directions. It is no source of new qualities. 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 quantities and degrees 

 of qualities, but not the qualities themselves. Species, 

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

 allies by quantities or by degrees their very qualities may 

 differ. 



From this discussion it may be seen that the slow and 

 gradual changes of ordinary variability and the production 

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

 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 variability 

 or fluctuation, since the individual qualities fluctuate around 



