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great advance in knowledge of the physiology and anatomy both of animals 

 and plants, that, for pm-poses of classification, there would be little difficulty 

 in defining the term " species " as applied to any particular class or group of 

 organisms, but this is far from being the case, for we find, that the most acute 

 and diligent enquirers, after careful study of the question, have acknowledged 

 that this term can only be applied arbitrarily and for the sake of convenience 

 merely, to some set of individuals closely resembling each other, and that in its 

 necessary application by systematists it does not essentially differ from the 

 term " vaxiety " which, in comparison with mere individual differences, is 

 usually given quite as arbitrarily to forms somewhat less distinct and more 

 fluctuating. 



This point has been made the subject of most elaborate investigation by 

 Mr. Darwin, in his volume on the " Origin of Species," in which he has 

 brought together a great mass of facts to prove that all classes of organisms 

 exhibit a greater or less degree of variability, and to show the consequent 

 difficulty of giving any conclusive or satisfactory definition of the term 

 " species," — the genei*al conclusions he arrived at being thus stated, — " Finally, 

 then, varieties have the same general character as species, for they cannot be 

 distinguished from species — except, firstly, by the- discovery of intermediate 

 linking forms, and the occurrence of such links cannot affect the actual char- 

 acters of the forms which they connect ; and except, secondly, by a certain 

 amount of difference, for two forms, if differing very little, are generally 

 ranked as varieties, notwithstanding that intermediate linking forms have not 

 been discovered ; but the amount of difference considered necessary to give to 

 two forms the rank of species, is quite indefinite. In genera having more than 

 the average number of species, in any country, the species of these genera have 

 more than the average number of varieties. In large genera the species are 

 apt to be closely, but unequally allied together, forming little clusters round 

 certain species. Species very closely allied to other species, apparently have 

 restricted ranges. In all these several respects the species of large genera 

 present a strong analogy with varieties. And we can clearly understand these 

 analogies, if species have once existed as varieties, and have thus originated : 

 whereas, these analogies are utterly inexplicable if each species has been 

 independently created. 



"We have, also, seen, that it is the most flourishing or dominant species of 

 the larger genera which, on an average vary most ; and varieties, as we shall 

 hereafter see, tend to become converted into new and distinct species. The 

 larger genera thus tend to become larger ; and, throughout nature, the forms 

 of life which ai*e now dominant, tend to become still more dominant by leaving 

 many modified and dominant descendants. But, by steps hereafter to be 

 explained, the larger genera also tend to break up into smaller genera. And 

 thus, the forms of life, throughout the Universe, become divided into groups 

 subordinate to groups." 



Flourens, in his paper on " La Quantite de vie sur le Globe," also tells us, 

 " that every species manifests two tendencies, namely, a tendency to vary and 

 a tendency to transmit to succeeding generations the acquired modifications ;" 

 and he, properly, observes, " that if the acquired variations and modifications of 

 each generation could not be transmitted to its descendants, such variations 

 and modifications would remain mere individual traits, and would never 

 become the chai'acteristics of a race." Flourens, however, does not appear to 

 have seen the full effect of these views as to the variability of species, for he 

 assumes that such variations only affect what he terms "superficial character- 

 istics," and that they are at all times easily detected, whilst he urges that "the 

 unity, identity, and reality of a species is always determinable by the presence 

 or absence of the moi-e deep seated characteristic of continuous fecundity." 



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