i io COMPARATIVE ANATOMY vin 



groups both of animals and plants. Until it is settled whether 

 there is an insensible blending in the conditions expressed by 

 the terms " variety," " race," " subspecies," " species," or whether 

 the old idea of the immutability of species is to be maintained, 

 zoology can hardly be said to have a philosophical basis. 



The second objection just named appears to suppose a 

 simultaneous march of all organic beings from form to form 

 a transmutation en masse of the whole so that to any one 

 among them they would all appear stationary. Such a view 

 is utterly opposed to the greater number of zoological and 

 palaeontological facts, and to all the necessities of the Dar- 

 winian hypothesis. One species, either with little inherent 

 capacity for variation, or so circumstanced that such variation 

 as may have occurred in different individuals has never been 

 accumulated by selection, may remain without alteration for 

 ages, while other species differently constituted or circumstanced 

 may have undergone vast transmutations during the same 

 period. In fact, the derivative hypothesis is not a theory of 

 things long ago ; it is not a curious speculation into the 

 beginnings of the present condition of things. It may be 

 appealed to every day in the solution of constantly occurring 

 problems in morphology. 



I will give an example from the subject of our present 

 course. There is in Australia an animal, the koala (Phascolarctos 

 cinereus), rather larger than a cat, but having more the aspect 

 of a small bear (Fig. 1). It is covered with a soft woolly fur, 

 and lives entirely among the boughs of trees, on the leaves 

 of which it feeds. Its tail is a mere rudiment. Its feet are 

 admirably adapted for grasping the branches on which it 

 climbs. The hind feet especially are remarkably modified for 

 this end, being very broad, and having a strong prehensile 

 or opposable inner toe or hallux, like a thumb. The skeleton 

 of that foot is very singular (see Fig. 2). The hallux is stout, 

 and placed nearly at a right angle with the other toes. The 

 next two digits are rather slender, placed close together, and 

 in the living animal are united almost to the claws in a 

 common integument. The two outer toes are free and much 

 stouter than the others, the fourth especially so. 



Now, a naturalist of the last century would have looked 



