CHAPTEK V 



HEREDITY AND VARIATION 



THE proximate cause of biological evolution is the- 

 appearance of variations. These arise during, 

 growth, and are transmitted from parent to offspring,, 

 so that the younger gradually diverge from the older 

 forms. This is called descent with modification. 



I will first describe shortly some of the principal 

 facts connected with these processes, and then give 

 the hypotheses which have been proposed to explain 

 them. 



Epitome of facts. Life is invariably associated 

 with the substance called protoplasm. This is a 

 semi-fluid material with a very complicated chemical 

 composition, but it always contains a considerable 

 quantity of water. It is not a definite compound , 

 for the proportion of its ultimate constituents varies. 

 And it is unstable only when it is alive and doing 

 work. This indeed necessitates its being unstable ; 

 for when work is going on , assimilation and secretion 

 must be going on also. But when these stop, pro- 

 toplasm is as stable a substance as any other. 

 Tinned meat keeps good for many years. 



According to the hypothesis of C. Nageli, all or- 

 ganic bodies are built up by a number of "micellae," 1 * 

 that is little groups of molecules surrounded by a 

 film of water ; and among these there float minute 



16 Called Elementary organisms by Briicke, Physiological 

 units by Herbert Spencer, Organic units by Galton, Plas- 

 tidules by Haeckel, and Biophores by Weismann. 



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