CHAPTER II. 



DEVELOPMENT.* 



50. CERTAIN general aspects of Development may bo 

 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 may be sure of finding them united by transitional modes. 

 But premising that a classification of modes can but approx- 

 imately represent the facts, we shall find our general con- 

 ceptions of Development aided by one. 



Development is primarily central. All organic forms of 

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



/ K 



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 de- 

 velopment, continues to be habitually shown in. the changes of 



* In ordinary speech, Development is often used as synonymous with Growth. 

 It hence seems needful to say, that Development as here and hereafter used, 

 means increase of structure, and not increase of bulk. It may be added, that the 

 word Evolution, comprehending Growth as well as Development, is to be reserved 

 foi occasions \vhenboth ,,vc implied 



