74 LIFE AND ORGANISATION. 



this cause is one not to be lightly given up to the demands 

 of a new doctrine, still unproved in some of its most essential 

 parts. 



The onus probandi, indeed, unquestionably rests here with 

 those who believe that species can undergo such transmuta- 

 tion, as permanently to change the conditions upon which 

 this distinction has naturally, or even necessarily, been 

 founded. It is their business to show some unequivocal 

 instances of perfect transmutation; or, in default of this, 

 some such approach to it, by gradations manifestly progres- 

 sive, as to warrant the presumption that time only is wanting 

 to complete the change. Less than this cannot be received 

 in evidence of fact, however plausible an hypothesis of pos- 

 sibilities may be made to appear. The limit-line drawn 

 around each species, by its power of self-reproduction, may not 

 be broken through without more complete proof than any yet 

 proffered to us. No unequivocal instance has hitherto been 

 obtained from any part of the animal kingdom to satisfy 

 fully these conditions. The question therefore remains 

 one of possibility and presumption only. Possibility can- 

 not be denied; but the advocate for the permanence of 

 species, resting upon much that is assured to his knowledge, 

 has a right to ask that the opposite doctrine should be fairly 

 fortified by fact before its admission even as one of the out- 

 works of science. 



The arguments for the hypothesis of transmutation are, 

 mainly, the variations which species actually undergo ; and 

 which in many cases, especially where man is the artificer of 

 new breeds, become fixed and hereditary ; the fact that in 

 a certain number of instances the intermixture of species is 

 prolific ; the existence of certain archetypes or general forms, 

 upon which the many specific forms are founded ; and the 

 tendency of all research, in the fossil as well as living world, 

 to bring the gradations between these forms into closer con- 



