CHAPTER II. 



DEVELOPMENT.* 



50. CERTAIN general aspects of Development may be 

 studied apart from any examination of internal structures. 

 These fundamental contrasts between the modes of arrange 

 ment of parts, originating, as they do, the leading external 

 distinctions among the various forms of organization, will be 

 best dealt with at the outset. If all organisms have arisen 

 by Evolution, it is of course not to be expected that such 

 several modes of development can be absolutely demarcated: 

 we are sure to find them united by transitional modes. But 

 premising that a classification of modes can but approximately 

 represent the facts, we shall find our general conceptions of 

 Development aided by one. 



Development is primarily central. All organic forms of 

 which the entire history is known, set out with a symmetri 

 cal arrangement of parts round a centre. In organisms of 

 the lowest grade no other mode of arrangement is ever 

 definitely established; and in the highest organisms central 

 development, though subordinate to another mode of develop 

 ment, continues to be habitually shown in the changes of 

 minute structure. Let us glance at these propositions in the 



* In ordinary speech Development is often used as synonymous with 

 Growth. It hence seems needful to say that Development as here and here 

 after used, means increase of structure and not increase of bulk. It may be 

 added that the word Evolution, comprehending growth as well as Develop- 

 uient, i3 to be reserved for occasions when both are implied. 

 102 



