358 MORPHOLOGY. [CHAP. XIV. 



orders, &c., \ve can understand the rules which we are compelled 

 to follow in our classification. We can understand why we value 

 certain resemblances far more than others ; why we use rudi- 

 mentary and useless organs, or others of trifling physiological 

 importance ; why, in finding the relations between one group and 

 another, we summarily reject analogical or adaptive characters, 

 and yet use these same characters within the limits of the same 

 group. We can clearly see how it is that all living and extinct 

 forms can be grouped together within a few great classes; and 

 how the several members of each class are connected together by 

 the most complex and radiating lines of affinities. We shall 

 never, probably, disentangle the inextricable web of the affinities 

 between the members of any one class; but when we have a 

 distinct object in view, and do not look to some unknown plan of 

 creation, we may hope to make sure but slow progress. 



Professor Hackel in his 'Generelle Morphologic' and in other 

 works, has recently brought his great knowledge and abilities to 

 bear on what he calls phylogeny, or the lines of descent of all 

 organic beings. In drawing up the several series he trusts chiefly 

 to embryological characters, but receives aid from homologous 

 and rudimentary organs, as well as from the successive periods 

 at which the various forms of life are believed to have first 

 appeared in our geological formations. He has thus boldly made 

 a great beginning, and shows us how classification will in the 

 future be treated. 



Morphology. 



We have seen that the members of the same class, inde- 

 pendently of their habits of life, resemble each other in the 

 general plan of their organisation. This resemblance is often 

 expressed by the term "unity of type;" or by saying that the 

 several parts and organs in the different species of the class are 

 homologous. The whole subject is included under the general 

 term of Morphology. This is one of the most interesting depart- 

 ments of natural history, and may almost be said to be its very 

 soul. What can be more curious than that the hand of a man, 

 formed for grasping, that of a mole for digging, the leg of the 

 horse, the paddle of the porpoise, and the wing of the bat, should 

 all be constructed on the same pattern, and should include similar 

 bones, in the same relative positions 1 How curious it is, to give 

 a subordinate though striking instance, that the hind-feet of the 

 kangaroo, which are so well fitted for bounding over the open 

 plains, those of the climbing, leaf-eating koala, equally well 

 fitted for grasping the branches of trees, those of the ground- 

 dwelling, insect or root eating, bandicoots, and those of some 

 other Australian marsupials, should all be constructed on the 



