AIM AND PROGRESS OF PHYSICAL SCIENCE. 387 



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, con- 

 cerning 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 origi- 

 nal marsupial, or, again, all vertebrates from a primitive 

 lancelet, or all plants and animals together from the slimy 

 protoplasm of a protiston, 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 different, 

 has already been traced in regularly deposited geological 

 strata, and has come to be beyond question ; and since 

 this line of research has been taken up, how numerous 

 are the facts which fully accord with Darwin's theory, 

 and give special effect to it in detail ! 



At the same time, we should not forget the clear in- 

 terpretation Darwin's grand conception has supplied of 

 the till then mysterious notions respecting natural affinity, 

 natural systems, and homology of organs in various 

 animals ; how by its aid the remarkable recurrence of 

 the structural peculiarities of lower animals in the 

 embryos of others higher in the scale, the special kind 

 of development appearing in the series of palseontological 

 forms, and the peculiar conditions of affinity of the faunas 



