AIM AND PROGRESS OF PHYSICAL SCIENCE. 339 



discussion ; and, in fact, this experiment has proved successful, 

 and is still doing so, with species of every class of the animal 

 kingdom, and, with respect to the most different organs of the 

 body, in a vast number of instances. 



After the general application of the law of transmission had 

 been established in this way, it only remained for Darwin to 

 discuss the bearings of the question as regards animals and 

 plants in the wild state. The result which has been arrived at 

 is that those individuals which are distinguished in the struggle 

 for existence by some advantageous quality, are the most likely 

 to produce offspring, and thus transmit to them their advan- 

 tageous qualities. And in this way from generation to genera- 

 tion a gradual adjustment is arrived at in the adaptation of each 

 species of living creation to the conditions under which it has to 

 live until the type has reached such a degree of perfection that 

 any substantial variation from it is a disadvantage. It will 

 then remain unchanged so long as the external conditions of its 

 existence remain materially unaltered. Such an almost abso- 

 lutely fixed condition appears to be attained by the plants and 

 animals now living, and thus the continuity of the species, at 

 least during historic times, is found to prevail. 



An animated controversy, however, still continues, concern- 

 ing the truth or probability of the Darwinian theory, for the 

 most part respecting the limits that should be assigned to the 

 variation of species. The opponents of this view would hardly 

 deny that, as assumed by Darwin, hereditary differences of race 

 could have arisen in one and the same species; or, in other words, 

 that many of the forms hitherto regarded as distinct species of 

 the same genus have been derived from the same primitive form. 

 Whether we must restrict our view to this, or whether, perhaps, 

 we venture to derive all mammals from one original marsupial, 

 or, again, all vertebrates from a primitive lancelet, or all plants 

 and animals together from the slimy protoplasm of a protis- 

 ton, depends at the present moment rather on the leanings of 

 individual observers than on facts. Fresh links, connecting 

 classes of apparently irreconcilable type, are always presenting 

 themselves; the actual transition of forms, into others widely 

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