MORPHOLOGY OF ANIMALS 



logical, a system of functions, but these never exist sepa- 

 rately. Changes in function inevitably involve changes in 

 structure and vice versa^ but neither is the cause of the 

 other, any more than one side of a coin is the cause of the 

 other side. Morphology and physiology are merely two 

 aspects of one thing, namely life. 



II. INDIVIDUALITY ' 



All living things from the simplest to the most complex are 

 distinct but not wholly independent individuals. 



1. Definition. An organic individual may be defined as any 

 unit capable of maintaining and of reproducing itself, or 

 in other words, capable of assimilation and reproduction. 



2. Grades of Individuality. The degree of differentiation 

 of any organism is directly proportional to the number of 

 unlike units that constitute it and to the degree of their 

 unlikeness. The following grades of organic individuals, 

 from the smallest and simplest to the largest and most 

 complex, are recognized: 



(<^) Ultra-microscopic units^ such as inheritance factors, 

 genes, etc. These invisible, but real units, have the power 

 of assimilation, growth, and division. 

 {b') Visible cell structures^ such as chromomeres, chromo- 

 somes, plastosomes, chromatophores, etc. These units also 

 assimilate, grow and divide; in both the ultra-micro- 

 scopic units (^) and these microscopically visible ones (Z?) 

 division is always into like portions, that is, it is non- 

 differential. 



(r) Cells are more complex units composed of the preced- 

 ing simpler ones (^a and Z?), each consisting of cytoplasm 



1:6: 



