xvin ORGANISATION AND EVOLUTION 415 



them as conjoint tests of " higher " organisation, and we 

 may define orthogenic evolution as the process by which 

 such organisation is attained. 



2. Now the organisation of the life of a species as a 

 whole depends on three more elementary forms of cor- 

 relation. These are : 



(i). The correlation of the constituent parts of the 

 individual perfection of structure and function. 



(2). The correlation of the several acts and experiences 

 of the individual. 



(3). The correlation of the acts and lives of different 

 individuals. 



With the two latter we have dealt. It remains to show 

 in a summary way how their development is associated 

 with an advance in physical organisation from the lowest 

 organisms to man. The Protista are classed as the lowest 

 organisms on account of their " simplicity," /.., the narrow 

 scope of their organisation, confined as it is to a single 

 cell. Among the lowest Metazoa, the unity of the distinct 

 cells that constitute the living being is so loose that it is 

 often difficult to say whether a certain aggregation of cells 

 should be regarded as one individual or many. 



"When from sundry points on the body of a common 

 polype, there bud out young polypes which, after acquiring 

 mouths and tentacles and closing up the communications between 

 their stomachs and the stomach of the parent, finally separate 

 from the parent ; we may with propriety regard them as distinct 

 individuals. But when in the allied compound, Hydrozoa, we 

 find that these young polypes continue permanently connected with 

 the parent ; and when by this continuous budding-out there is 

 presently produced a tree-like aggregation, having a common 

 alimentary canal into which the digestive cavity of each polype 

 opens ; it is no longer so clear that these little sacs, furnished with 

 mouths and tentacles, are severally to be regarded as distinct 

 individuals. We cannot deny a certain individuality to the 

 polypedom." * 



Long after organisation is sufficiently developed to leave 

 no doubt as to the unity of the organism, the separate 



1 Herbert Spencer, Principles of Biology, Vol. I. p. 246. Cf. Verworn, 

 pp. 63, 64. 



