^ 



420 



CONCLUDING EEMAEKS. 



Chap. XXVIII 



conjecture what is the nature of the bond of 



But 



between homologous 



between the fore and hind limbs 



between the hair, hoofs, horns, and teeth 



that parts 



which are 



losely 



il 



during their early develop! 



and which are exposed to similar conditions, would be liable 



b 



modified in the same manner. Homologous p 



fr 



ha vino- the same nature, are apt to blend together, and, when 

 many exist, to vary in number. 



Although every variation is either directly or indirectly caused 



A in the surrounding: conditions, we must never 



by some chan_ 



forget that the nature of the organisation which is acted on essen- 

 tially governs the result. Distinct organisms, when placed under 

 similar conditions, vary in different manners, whilst closely-allied 

 organisms under dissimilar conditions often vary in nearly the 

 same manner. We see this in the same modification frequently 



time in the same variety, and 

 likewise in the several striking cases given of analogous or paral- 



appeai 



Ion ix intervals of 



Although some of these latter cases are simply 



lei 



due to reversion, others cannot thus be accounted for. 



From the indirect action of changed conditions on the or- 

 ganisation, through 



the impaired state of the reproductive 



K 



from the direct action of such conditions (and 



cause the individuals of 



species either to vary 



same manner, or differently in accordance with slight difference! 

 in their constitution)— from the effects of the increased or de 



creased use of parts,— 



* 



our domesticated produ 



and fr 



is complicated 



ability of 

 an extreme 



Al- 



degree. The whole organisation becomes slightly plast 

 though each modification must have its proper exciting cause, 

 and though each is subjected to law, yet we can so rarely trace 

 the precise relation between cause and effect, that we are tempted 

 to speak of variations as if they spontaneously arose, 

 even call them accidental, but this must be only in the sense 



say that a fragment of rock dropped from a height 



We may 



shap 



It mav be worth while briefly to consid 



the results of 



exposure to unnatural conditions of a large number of animals o 

 the same species, allowed to cross freely, with no selection of any 





•. 



