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THE ZOOLOGIST. 



primary forms, he puts aside the ordinary view of the indepen- 

 dent creation of each organism, and proceeds to give his 

 reasons for this conviction. Briefly stated, Darwin's Theory of 

 Natural Selection supposes : — (1) That every kind of animal and 

 plant tends to increase in number in a geometrical progression. 

 (2) That each tends to transmit a general likeness with in- 

 dividual differences to its offspring. (3) Every individual may 

 present minute variations of any kind and in any direction. 

 (4) Past time has been practically indefinite. (5) Every in- 

 dividual has to endure a severe struggle for existence. (6) Every 

 variety of a kind tending to save the life of the individual 

 possessing it, or to enable it more surely to propagate, will in 

 the long run be preserved. He first illustrates the variability 

 of animals by appealing to what we find in domestic species. 

 Every one is familiar with the numerous races of many domesti- 

 cated animals, such as the horse, ox, sheep, dog, and pigeon, 

 and moreover of the difficulty which one would have in saying 

 whether a given race, say, of cattle, is merely a variety or a 

 distinct species of cattle. Even Bacon long ago wrote in regard 

 to variability by domestication : " By art also we make them 

 greater and taller than their kind is"; "Also we make them 

 differ in colour, shape, activity, many ways." The extraordinary 

 variability, for example, of domestic pigeons is striking, for it 

 is generally admitted that they are probably all derived from a 

 single species of pigeon — the Bock-Pigeon (Columba livia). Any- 

 one, even a naturalist, who for the first time surveys specimens 

 so widely different as these varieties, would at once call each a 

 separate species, and he might further group some under diffe- 

 rent genera ; that is, their distinctions are so great that there 

 would be even more than specific divergence. It would, in- 

 deed, be hard to convince him that these were not produced 

 originally from as many separate pairs of pigeons or species as 

 there were varieties. Yet such cannot be the case, for, besides 

 the statement previously made, only two or three other species 

 of Bock-Pigeon exist, and these have none of the characters of 

 the domestic breeds. That the numerous varieties of domestic 

 pigeons are artificial productions, if the term may be used, their 

 very curious structure of beak, feathers, and crop sufficiently 

 testifies, for in no other group of pigeons is anything similar to 



