THE MUSEUM 



265 



uals so nearly alike in character that 

 they may be supposed to have de- 

 scended from a pair having like char- 

 acter with themselves and capable of 

 perpetuating their kind, is perhaps the 

 usual idea of what constitutes a spec- 

 ies. And this seems to rest upon the 

 two notions of resemblance and hered- 

 it\'. Both these notions, however, are 

 true only within certain limits. The 

 amount or kind of resemblance that 

 rightly constitutes individuals into a 

 species is not a ti.xed, precise or meas- 

 urable quantity. Any difference, how- 

 ever small, between two individuals, 

 provided the difference is constant and 

 continuously reappears in the offspring 

 will make the two individuals of differ- 

 ent species. Because the insensibly 

 graded links in the chain of certainty, 

 if there be such links at all, are not 

 constant within assignable limits, and 

 the very application of the terms 

 inconstant and variable makes them 

 varieties. Specific differences must 

 e.xist apart from, and without any in- 

 fluence exerted by, man's aid or care. 

 While by selection and care we may 

 make almost any variety of plant 

 transmissible by seed and cause it to 

 become a strongly marked variety — a 

 race — yet races cannot be said to exist 

 independently of man; and he really 

 does not produce them. Such pecu- 

 liarities occasionally originate from un- 

 known causes and are preserved and 

 more fully developed by the cultiva- 

 tion and skill of man; and without this 

 dwindle and perish, or resort to the 

 original form of the species. (Gray.) 

 The Harp shells, usually reckoned 

 as consisting of twelve species, are by 

 some good naturalists considered as 

 only one; although the difference is 

 considerable in resemblance. It is 



hard to find two shells of Coiumbella 

 fulgurans, or of Nerita peleronta and 

 many other species that pass as well- 

 defined and undoubted species, that 

 have the same color markings. The 

 same is true of Purpura crispata, both 

 in regard to color and external mark- 

 ings; and many others might be men- 

 tioned presenting as great or greater 

 diversity. 



We are therefore led to believe that 

 in species there is room for variation 

 within certain limits that are never 

 over-stepped; and that the descendant 

 is liable at any time to resort to the 

 original type, and will do so sooner or 

 later. In this there is great analogy 

 to the revolutions of the heavenly bod- 

 ies in their orbits. The planets are 

 continually varying their position as 

 they course around the sun, yet there 

 is that powerful, controlling influence 

 of gravitation, that brings them back 

 again to their starting place. The 

 comet may shoot off into space seem- 

 ingly never to return, but the same 

 law brings it back. May there not be 

 an unknown law controlling the living 

 world, that even in these forms, that 

 like the comet, seem to be going off 

 by variability from all ancestral rela- 

 tionship never to return, just as surely 

 brings them back, as the comet was 

 forced to return by the law of gravita- 

 tion.' As we know there is a great 

 space between the planets of our sys- 

 tem, and a still greater distance before 

 the next system of worlds is reached, 

 so there is an assignable and time 

 space in kind between species, and a 

 yet greater when we attempt to pass 

 from one genus — which is only another 

 name for a grouped system of species 

 — to another. 



Variation is most marked in domes- 



