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tion. Such, variations as have been witnessed or 

 made the subject of special research have taken place 

 within limited periods, and under artificial aids and 

 sameness of conditions ; whereas in nature variation 

 takes place concomitantly with change ia conditions, 

 and is thus subjected Ulimitably to those forces 

 which continue to favour its divergence. This 

 knowledge of variation in existing nature, and the 

 discovery by paleeontology that the lower forms of 

 life are gradually succeeded by the higher — the 

 invertebrate by the vertebrate, the fish by the 

 reptUe, the reptile by the bird, and the bird by 

 the mammal — has given rise to what is generally 

 known as the Development Hypothesis, which seeks 

 to connect the whole scheme of life by a process of 

 derivative descent, just as it is evidently bound to- 

 gether by one great structural conception.* In work- 



* The Development Hypothesis, as entertamed by its leading 

 supporters, may be briefly enunciated as follows : — 1. That all the 

 genns of future plants, organical bodies of aU kinds, and the repro- 

 ducible parts of them, were really contained in the first germ ; 

 2. That species were not produced by independent creation, but that, 

 under operation of a general law, the germs of organisms produced 

 new forms different from themselves when particular circumstances 

 called the law into action ; and 3. That these evoking circumstances 

 have occurred in definite order, and in conformity with a great pre- 

 ordained plan, whereby the scheme of life has ever been kept in 

 harmony with the ordinal rank which now prevails among plants 

 and animals. 



