433 PROFESSOR LANKESTER. 



VI. The more general Causes and Modes of 

 Development. 



In the preceding sections of this essay we have discussed 

 the probable succession of forms and the particular phases 

 of increasing complexity which animal organisms have pre- 

 sented in the course of their historical development, and an 

 attempt has been made to show that the phenomena of 

 individual development from the egg may be considered as 

 more or less slurred and interrupted recapitulations or 

 epitomes of the historic development. 



Now, let us take a more general point of view, and endea- 

 vour to state xohat are the more general causes or antecedents 

 of organic development, and what the more general effects of 

 those causes, that is to say, the modes of their operation ; so 

 we shall come nearer to the ultimate goal of biology which 

 is the accounting for the phenomena of living matter or 

 protoplasm by reference to the laws of chemistry and 

 physics. 



In order to look at development from the physiological 

 point of view it is necessary to take a glance at the structure 

 of organisms in relation to their activities. 



The following propositions contain the essential doctrine 

 of the interdependence of structure and function. 



1. Every organism is either a single corpuscle of pro- 

 toplasm or an aggregate of such corpuscles, variously 

 modified. 



2. A corpuscle of protoplasm or " unit of organic structure" 

 is called a plastid. A plastid wbich possesses a differentiated 

 kernel or nucleus is called a cell ; one devoid of nucleus is 

 called a cytod. 



3. The living substance of all organisms, whether con- 

 sisting of many or of one single plastid, exhibits the following 

 activities, which are explained by its chemical and physical 

 constitution : — 1. Contractility. 2. Irritability. 3. Re- 

 ception and assimilation of foreign matter. 4. Chemical 

 change and secretion. 5. Respiration, e. e. combination with 

 oxygen and excretion of carbonic acid. 6. Reproduction, 

 either resulting in growth or, when accompanied by self- 

 division, in multiplication of individuals. 



4. In the lowest organisms, which consist of one single 

 plastid, these various activities are carried on by one and the 

 same corpuscle of protoplasm. In the higher organisms, 

 consisting of many plastids, they are exhibited more or less 

 clearly by each and all, but are intensified variously in par- 

 ticular plastids. According to their position and the particular 



