EVIDENCE FURNISHED BY SCIENCE OF LIKENESSES. 129 



"adaptation" forms, along with descent, a factor of no slight im- 

 portance in modifying the structures of living beings. Every living 

 thing is subject to the perpetual and continuous action of its environ- 

 ments or surroundings. Such outward influences may favour or 

 retard the evolution and growth of new parts and organs, and will 

 unquestionably induce now, as in the past, alterations in the struc- 

 ture and form of the living being. Of the exact influence and extent 

 of the external causes of variation we know very little, but of the 

 existence of such causes no one entertains a doubt. The question, 

 however, presents itself as to the nature of the likenesses and differ- 

 ences which such outside- influences may produce. All likenesses or 

 homologies which cannot be accounted for on the theory of descent 

 from a common ancestor are named " homoplastic," according to 

 Mr. Ray Lankester's terminology. As an example of both kinds of 

 likeness, it may suffice to cite the limbs and heart of higher verte- 

 brata and the swimming-bladder of fishes, as illustrative of " homo- 

 genous " parts, or those which are the products of inheritance. The 

 heart of a bird and a quadruped are " homogenous " organs, but 

 the cavities or compartments are " homoplastic," or, in other words, 

 have been developed independently of each other, as, in all proba- 

 bility, have the feathers of the one and the hairs of the. other. It 

 is well, therefore, to take into account this false or incomplete 

 " likeness," which expresses no blood-relationship, and which, in its 

 production, involves much that is obscure. We can explain the 

 likeness between limbs on the theory of descent from a common 

 type ; the likeness between a worm and a lobster,. in respect of their 

 jointed bodies, becomes clear on this theory ; but we cannot so 

 account for the close likeness between the individual joints of a worm, 

 or between those of a lobster, or, for that, between the feelers, jaws, 

 and feet of the latter animal, on the principle of inheritance. Mr. 

 Darwin says : the formation of such structures " may be attributed 

 in part to distinct organisms, or to distinct parts of the same 

 organism, having varied in an analogous manner ; and in part to 

 similar modifications having been preserved for the same general 

 purpose or function." 



Leaving, as still under the shadow of unapprehended causes, the 

 variation of parts from outward forces operating upon the living 

 being and its structure, let us turn to some clear examples of plain, 

 though at first sight unapparent, " likenesses." which may be drawn 

 from both animal and plant kingdoms. Our examples may comprise 

 a wide range of subjects ; but this facility of illustration is in itself 

 a proof of the universal application of the science of likeness to 

 explain the modifications of common types through which the forms 

 of life have come to exhibit that diversity which is at once the 

 wonder and the charm of living nature. 



K 



