n6 COMPARATIVE ANATOMY vin 



conditions of existence to which the animal is exposed, or, in 

 other words, adaptive characters, is a constantly recurring 

 problem to the systematic morphologist ; and the difficulties 

 that encompass the solution of this problem are the main 

 causes of the little progress hitherto made towards a general 

 agreement as to classification otherwise than in the main 

 groups. Let me illustrate this point by a single and, as it 

 happens, by no means difficult example. 



You have before you the skeletons of two animals very 

 similar in their general or superficial characters one is the 

 common dog, the other the carnivorous marsupial, the thylacine, 



FIG. 5. The Thylacine (Thylacinus cynocephalus). From Wolf s Zoological 



Sketches. , 



sometimes called " Tasmanian wolf." These animals, when wild, 

 have very similar habits. You will see that, in the general 

 characters of the skeleton, the structure of the limbs, and the 

 arrangement of the teeth, a remarkable similarity prevails. You 

 might easily conclude that these animals were related. They 

 were, indeed, formerly placed near together by systematists. 



Now, if I place by the dog another well-known animal, a 

 sheep, you will see that in many characters of its bones, its 

 feet, its teeth, etc., it differs far more from the dog than the 

 latter does from the thylacine. Yet zoologists affirm, without 

 the slightest hesitation, that the sheep and the dog are far 

 more nearly related to each other than either is to the thylacine. 

 This assertion is founded on a discrimination of the essential 

 from the adaptive characters. The dog and the sheep belong 



