NOTES ON EMBRYOLOGY AND CLASSIFICATION. 433 



activity which they pre-eminently exhibit, the plastids of 

 such an organism vary in form, and the nature and amount 

 of chemical change to which the protoplasm composing them 

 has been subject varies also. They may be spherical, colum- 

 nar, prismatic, scale-like, star-shaped, spindle-shaped (fusi- 

 form), branched, or united to form fibres. The substance 

 between the plastids may be absent (when the plastids are 

 continuous), very small in amount or large in amount. It 

 may be solid and dense, or gelatinous and viscous, or 

 fibrous or liquid. 



5. When a number of plastids exhibiting pre-eminently 

 one kind of activity (see sec. 3), and having one particular 

 form and one particular kind of intermediate substance (if 

 any) are found forming a layer or separable tract in an 

 organism they are said to constitute a tissue. 



6. In a higher organism the activities enumerated in sec. 3 

 (each of which is exhibited more or less by each of its con- 

 stituent plastids) are carried on through the agency of a 

 number of specially adapted parts, which are known as 

 ORGANS. Every organ has its function in relation to one 

 or other of these activities. Many tissues may enter into the 

 composition of an organ. A series of connected organs forms 



a SYSTEM. 



7. The development of organisms (of which in the earlier 

 part of this essay we have traced the concrete expression, so 

 far as relates to animals) is primarily caused by the advan- 

 tage gained for an organism in the struggle for existence by 

 the distribution of the activities specified in sec. 3 — amongst 

 special parts, that is to say, by the advantages gained by 

 the possession of elaborate tissues and organs. 



8. The possibility of development is solely due to the 

 physico-chemical constitution of protoplasm, in virtue of 

 which constitution it is subject to (a) unlimited Variation by 

 the action of incidental forces, and to (b) the permanence of 

 impressions or Memory. In virtue of the memory of proto- 

 plasm, fortunate variations, acquired in the life-and-death 

 conflict with the environment, h^comQ permanent adaptations 

 in such organisms as survive (survive as individuals or as new 

 generations). Those with unfortunate variations die off in 

 consequence of the want of fitness of their variations (both 

 newly acquired and constitutional) to the conditions pre- 

 sented by the struggle for existence. Thus the selection of 

 new variations, and the constant accumulation of old- 

 acquired improvements by the operation of the protoplasm's 

 memory in the survivors of the struggle for existence goes on. 

 The elaborate distribution of functions amongst specialised 



