EXPLANATOKY HYPOTHESES 101 



would accumulate. Habits also were inherited and 

 became instincts. The outgrowth of new parts he 

 attributed to the flow of nutrient matter to the part 

 when irritated. 



Mr. Herbert Spencer, in his " Principles of 

 Biology " (1864), proposed the hypothesis of 

 " Organic Polarity," which he denned as "a pro- 

 clivity towards a specific structural arrangement," 

 and said that it " results from the proclivity which 

 the component units, contained in a germ-cell or a 

 sperm-cell, have to arrange themselves into a struc- 

 ture like that of the structure from which they were 

 derived." Variations, he thinks, may be due either 

 to the action of external causes (now called physio- 

 genesis) or to spontaneous variation, due to 

 differences in the ova , no two of which can be exactly 

 alike ; or to the mixture of characters in sexual repro- 

 duction (what is now called amphimixis). And he 

 further says (2nd ed., vol. I., p. 189), "The one 

 ultimate principle that in an organism equal amounts 

 of growth take place in those directions in which the 

 incident forces are equal, serves as a key to the 

 phenomena of morphological development." This 

 hypothesis is now usually called Epigenesis, and Dr. 

 Hertwig is its chief exponent. It supposes that 

 growth and development are entirely dependent on 

 outside physical agencies, and that they are entirely 

 controlled by the environment. Mr. Spencer has 

 put the theoretical view of variation by physiogenesis 

 very clearly. He says (2nd ed., vol. I., p. 351) 

 "While an aggregate of physiological units continues 



