222 OUTLINES OF EVOLUTIONARY BIOLOGY 



sharply circumscribed groups. A child collecting flowers by 

 the wayside will tell you without difficulty how many kinds he 

 has found, and these kinds speaking generally correspond 

 to the different species recognized by the botanist. 



Even amongst the higher and better known plants and 

 animals, however, the so-called species are not always sharply 

 distinguishable from one another, for we not infrequently meet 

 with intermediate forms, or connecting links, between what are 

 usually recognized as distinct species. Linnaeus himself, who 

 pronounced most explicitly in favour of the doctrine of special 

 creation, 1 was forced by his profound knowledge of the animal 

 and vegetable kingdoms to recognize this fact in the case of 

 certain groups of plants 2 ; and when we come to study some of 

 the lower members of the animal kingdom, as, for example, the 

 sponges, the difficulty of distinguishing species sharply from one 

 another frequently becomes so great that the definition of any 

 particular one is little more than a matter of individual 

 opinion. The more we study the animal and vegetable king- 

 doms, in short, the more clearly is the fact impressed upon us that 

 if we could have before us all past and present individuals we 

 should find it impossible, except in an arbitrary manner, to 

 arrange them in species at all, for each kind would be found 

 to be connected with others by a series of small gradations. 



According to Darwin, the differences which separate existing 

 species from one another have, at any rate in most cases, arisen 

 by the gradual accumulation of small successive variations. If 

 this be the case the fact that we are able logically to distinguish 

 such species from one another at all can only be due to that dis- 

 appearance of the older portions of the organic tree whereby 

 discontinuity has arisen between the surviving branches. In 

 any case it is quite certain that such destruction of former 

 connecting links has played a very large part in the separation of 

 those groups of individuals to which the term species is usually 

 applied. This process is clearly illustrated in the accompanying 

 diagram (Fig. 85), in which each enclosed area is supposed 



1 " Species tot sunt, quot diversas formas ab initio prodnxit Infinitum Ens ; quas 

 cleinde formae secundum gencrationis inditas leges produxere plures, at sibi semper 

 similes, ut Species mine nobis non sint plures, quam qua3 fuere ab initio." 



Linnseus. Gen. Plant. (1737). 



2 " Species Rosarum difficillime limitibus circumscribuntur et forte natura vix 

 cos posuit." Liimams. Spec. Plant,, Ed. 2, p. 705 (17(52). 1 am indebted to my 

 friend and colleague at the Linnean Society, Dr. B. Daydon Jackson, for showing 

 me these quotations. 



