3'JO MORPHOLOGY. Chap. XIII. 



ment of ilrst'cnt in chissint!^ the in(livi<luals of both sexes and 

 of all ag"C3 imjcr one species, altliougli they niay hare but few 

 cliaracters in common ; avc use descent in chissing acknowl- 

 ed<:;ed varieties, liowever different they may be from tlicir par- 

 ent ; and I believe this element of descent is the hidden bond 

 of connection -wliich naturalists have sought under the term of 

 the Natural System. On this idea of the natural SA'stem being, 

 in so far as it has been perfected, genealogical in its arrange- 

 ment, with tlie grades of difference exjoressed by the terms 

 genera, families, orders, etc., we can understand the rules 

 Avhich we are compelled to follow in our classification. We can 

 imderstand why we value certain resemblances far more than 

 others ; why we use rudimentary and useless organs, or others 

 of trifling physiological importance ; why, in finding the rela- 

 tions between one grouji and another, we summarily reject 

 analogical or adaptive characters, and yet use the same char- 

 acters, within the limits of the same group. Wc can clearly 

 see how it is tliat all living and extinct forms can be grouped 

 together within a few great classes; and how the several mem- 

 bers of each class are connected together l)y the most complex 

 and radiating lines of affinities. We shall never, probably, 

 disentangle tlie 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 luiknown plan of creation, 

 we mav hojie to make sure but slow jirogress. 



Prof. Iliickel, in his " Generelle Morphologic " and in sever- 

 al 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 se- 

 ries he trusts chiefly to embryological characters, but draws aid 

 from homologous and rudimentary organs, as well as from the 

 successive periods at which the various forms of life first ap- 

 peared in our geological formations. He has thus boldly made 

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

 future l)e treated. 



Morphology, 



AVe have seen that the members of the same class, indepen- 

 dently of their habits of life, resemble each other in the general 

 j)lan of their organization. This resemblance is often expressed 

 by the term " unity of t>*])e ; " or by saying that the several 

 parts and organs in the different species of the class are homol- 



