ix CAUSES OF BELIEF IN FIXITY OF SPECIES 125 



That they often vary a little cannot escape careful observation, 

 but the deviations from the common characters of the kind to be 

 noticed by persons whose range of vision is thus limited are not 

 striking, and usually appear not to pass beyond certain bounds. 

 Hence arose the common idea, natural enough under such 

 circumstances, but which gradually developed itself, not only 

 into a scientific hypothesis, but even, it would appear, almost 

 into an article of religious belief, that the different kinds or 

 " species," as they are technically called, of animals and plants, 

 had each its separate origin, its fixed limits of variation, and 

 could not under any circumstances become modified or changed 

 into any other form. 



This idea became deeply rooted in the human mind, in 

 consequence of the very long period during which it prevailed, 

 the horizon of observation having remained practically 

 stationary from the time man first began to observe and record 

 the phenomena of nature, until little more than a century ago, 

 when commenced that sudden expansion of knowledge of the 

 facts of the animal and vegetable world, which has been 

 steadily widening ever since. Now it is important to observe 

 that it is strictly pari passu with the growth of knowledge of 

 the facts, that the theoretical views of nature have changed, 

 and the older hypothesis of species to which I have referred 

 has gradually given way to a new and different one. 



The expansion of the special branches of knowledge affect- 

 ing our views upon this subject has taken place in many 

 different directions, of which I can here only indicate the 

 most striking. 



1. The discovery of enormous numbers of forms of life, the 

 existence of which was entirely unknown a hundred years ago. 

 The increase of knowledge in this respect is something incon- 

 ceivable to those who have not followed its progress. Not 

 only has the number of well-defined species known multiplied 

 prodigiously, but infinite series of gradations between what 

 were formerly supposed to be distinct species are being con- 

 stantly brought to light. The difficulty of giving any satis- 

 factory definition of what is meant by the term " species " 

 is increasingly felt day by day by practical zoologists, as 

 evidenced by the introduction of such terms as " sub-species," 



