ON VARIETIES, RACES, SUB-SPECIES, AND SPECIES. ] 07 



ascertain whether the phenomena and effects we observe are 

 really attributable to what we have conceived to be their prox- 

 imate causes. We think our object attained when we are certain 

 that the effect is proportional to the intensity of its cause or of 

 the force to which it is referred, because it is possible to express 

 this proportion numerically. It is for these reasons that we do 

 not admit the existence of sciences of pure observation and 

 reasoning, but sciences of observation, reasoning, and experiment 

 combined, because where there is no experiment there is nothing 

 but conjecture, or at most induction.* 



Our definition of species is subordinate to two general facts ; 

 the greatest possible resemblance in the organization of the in- 

 dividuals composing it, and the transmission of this form from 

 generation to generation. 



We have not considered this transmission as absolute, but as 

 depending on the circumstances in which the individuals live ; 

 as we cannot ascertain more than the effects of causes in opera- 

 tion at the present time, Ave have admitted the persistence of the 

 essential nature of species at least since the last revolutions in 



* If we wished to justify our preference of the experimental method in 

 the study of natural history, we should only have to refer to the history of 

 the genus Medusa, for which science is indebted, first to MM. Sars and 

 Siebold. and then to Van Beneden and M. J. Dujardin. 



The female medusa lays eggs, which are fecundated by a male medusa 

 under circumstances still unknown. 



These eggs yield larvaj with vibrating ciliie, and which would certainly 

 be taken for infusoria if they were examined in an isolated way. 



These larvie change into water polyps (polypes hydraires), which either 



(a) Divide into segments, which segments become medusae ; or 

 (6) Produce — 



1. By gemmation other water polyps, which remain aggregated 



together. 



2. Bulbils, which separate from the polyp and produce aggre- 



gated polyps, as in the last case. 

 The aggregated polyps forming medusae in both cases. 

 Before the researches of the gentlemen above named, the larvae of medusw, 

 their polyps, and medusae in a perfect state, belonged to the last three classes 

 of zoophytes, according to Cuvier's classification, thus:— 

 The larvcE to the 5th class, viz. infusoria. 

 The polyps to the 4th class, viz. polypi. 

 The meduscE, in their perfect state, to the 3rd class, viz. acalepha. 



As we have already stated, it is only by experiment that we can hope to 

 ascertain with certainty the transmission of form from ascendants to de- 

 scendants, and thus, too, the precise definition of any given species. We 

 should never have found out, except by experiment or observation, that an 

 animal, like a plant, could be propagated by the division of a still imperfect 

 individual, and again by generation when the same individual is fully or- 

 ganized. 



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