288 



Scientific Intelligence. 



reply that, were I asked whether genera had a real existence in the ani- 

 mate world, my answer would be that they undoubtedly have, — though 

 not in the sense (which is so commonly supposed) of abrupt and discon- 

 nected groups. I conceive them to be gradually formed nuclei, through 

 the gathering together of creatures which more or less resemble each 

 other, around a central type : they are the dilatations (to use our late 

 simile) along a chain which is itself composed of separate, though dif- 

 ferently shaped links, — the links being the actual species themselves, 

 and the swellings, or nodes, the slowly developed genera into which they 

 naturally fall. When I say " slowly developed," my meaning may pos- 

 sibly require some slight comment. It is simply therefore to guard 

 against the fallacy, which I have so often disclaimed, that genera are 

 abruptly (or suddenly) terminated on their outer limits, that the expres- 

 sion has been employed. Though I believe that a series of species, each 

 partially imitating the next in contact with it, is Nature's truest system ; 

 yet we must be all of us aware that those species do certainly tend, in 

 the main, to map out assemblages of divers phases and magnitudes, dis- 

 tinguished by peculiar characteristics which the several members of each 

 squadron have more or less in common. So that it is ttfily in the middle 

 points that these various groups, respectively, attain their maximum, — 

 every one of which (by way of illustration) may be described as a con- 

 centric bulb, which becomes denser, as it were, in its successive compo- 

 nent layers, and more typical, as it approaches its core." 



The main topic of the work is the variations which species undergo. 

 He illustrates it by facts and urges the importance of its study as the 

 foundation of our knowledge of species. With every species in nature, 

 organic or inorganic, there appears to be a normal type admitting of 

 librations in many of its characters, on either side through external influ- 

 ences ; and the complete idea involves a knowledge of the extent and 

 laws of these librations. We cite the following from the author's con- 

 cluding chapter. 



" As regards that most obscure of questions, what the limits of species 

 really are, observation alone can decide the point. It frequently hap- 

 pens indeed that even observation itself is insufficient to render the 

 lines of demarcation intelligible, — therefore, how much more mere dia- 

 lectics ! 



To attempt to argue such a subject on abstract principles, would be 

 simply absurd ; for as Lord Bacon has remarked, the " subtility of Na- 

 ture far exceeds the subtility of reasoning :" but if, by a careful collation 

 of facts, and the sifting of minute particulars gathered from without, 

 the problem be fairly and deliberately surveyed, the various disturbing 

 elements which the creatures have been severally exposed to, having 

 been duly taken into account, the boundaries will not often be difficult 

 to define. Albeit, we must except those races of animals and plants 

 which, through a long course of centuries, have become modified by 

 man, — the starting-points of which will perhaps continue to the last to be 

 shrouded in mystery and doubt. It would be scarcely consistent indeed 

 to weigh tribes which have been thus unnaturally tampered with by the 

 same standard of evidence as we require for those which have remained 

 for ever untouched and free,-— especially so, since (as we have already 



