396 THE ZOOLOGIST. 



Isotely may arouse unsuspected correlations and thus bring ever so 

 many more homoplasies in its wake. 



Function is always present in living matter ; is its life. It is 

 function which not only shapes but creates the organ or suppresses 

 it, being indeed at bottom a kind of reaction upon some stimulus, 

 which stimuli are ultimately all fundamental, elementary forces, 

 therefore few in number. That is a reason why Nature seems to 

 have but few resources for meeting given " requirements " — to use 

 an everyday expression which really puts the cart before the horse. 

 This paucity of resources shows itself in the repetition of the same 

 organs in the most different phyla. The eye has been invented 

 dozens of times. Light, a part of the environment, has been the 

 first stimulus. The principle remains the same in the various eyes ; 

 where light found a suitably reacting material a particular evolution 

 was set going, often round about, or topsy-turvy, implying amend- 

 ments ; still, the result was an eye. In advanced cases a scientifically 

 constructed dark chamber with lens, screen, shutters, and other 

 adjustments. The detail may be unimportant, since in the various 

 eyes different contrivances are resorted to. 



Provided the material is suitable, plastic, amenable to prevailing 

 environmental or constitutional forces, it makes no difference what 

 part of an organism is utilised to supply the requirements of function. 

 You cannot make a silk purse out of a sow's ear, but you can make 

 a purse, and that is the important point. The first and most obvious 

 cause is function, which itself may arise as an incidental action due 

 to the nature of the material. The oxyclising of the blood is such a 

 case, and respiratory organs have been made out of whatever parts 

 invite osmotic contact of the blood with air or water. It does not 

 matter whether respiration is carried on by ecto- or by endodermal 

 epithelium. Thus are developed internal gills, or lungs, both of 

 which may be considered as referable to pharyngeal pouches : but 

 where the outer skin has become suitably osmotic, as in the naked 

 Amphibia, it may evolve external gills. Nay, the whole surface of 

 the body may become so osmotic that both lungs and gills are 

 suppressed, and the creature breathes in a most pseudo-primitive 

 fashion. This arrangement, more or less advanced, occurs in many 

 Urodeles, both American and European, belonging to several sub- 

 families, but not in every species of the various genera. It is there- 

 fore a case of apparently recent Isotely. 



There is no prejudice in the making of a new organ except in so 

 far that every organism is conservative, clinging to what it or its 

 ancestors have learnt or acquired, which it therefore seeks to re- 

 capitulate. Thus in the vertebrata the customary place for respi- 

 ratory organs is the pharyngeal region. Every organism, of course, 

 has an enormous back history ; it may have had to use every part in 

 every conceivable way, and it may thereby have been trained to such 

 an extent as to yield almost at once, like a bridle-wise horse, to some 

 new stimulus, and thus initiate an organ straight to the point. 



Considering that organs put to the same use are so very often the 

 result of analogous adaptation, homoplasts with or without affinity of 



