THE CorKSK OF BIOLOGIC KVOLl'TION. 25 



7y/>cs of Structure. In studying the operation of this law 

 in biology a number of important facts are to be noted. It is 

 first necessary to consider what may be called types of structure. 

 These, in our illustration of sympodial dichotomy, represent 

 first, the main trunk, and afterward the successive branches 

 which become virtually the main trunk. Except in parasitism 

 and other anomalous cases, the development along the main 

 trunk is in the nature of an ascending series of forms, in the 

 sense that the structure grows more and more perfect. There 

 is a differentiation of organs and functions and an integration 

 of parts into organisms of higher and higher capacity, but all 

 are constructed upon the same general plan and represent a 

 single and uniform type. This process of perfectionment in 

 the organisms of original type constituting the main trunk 

 proceeds as far as the nature of that type of structure will 

 permit. The branch which is to constitute the new and higher 

 type is ultimately developed out of this original trunk, but 

 there is no fixed time for its appearance. The original type 

 may have reached its maximum development and remained 

 stationary for any length of time, or it may have already be 

 gun to decline before this takes place. In fact it may never 

 take place, but such organisms perish and leave us no history. 

 The branch must possess a higher type of structure, otherwise 

 it must dwindle and also disappear. To give it fresh vigor and 

 power to continue the stock it must have acquired, through the 

 known laws of variation and selection, some advantageous char 

 acter not possessed by the original type, to which its superior 

 vigor is due. It then in turn continues to develop and goes on 

 improving in the same manner as the main trunk did before it, 

 until, like that, it reaches the maximum limit to its capacity 

 for progress, i. <?., until nothing more can come from that type 

 of structure. Like its ancestor, too, it then remains stationary 



