EVOLUTION. THE MASTER-KEY 



cell-size is such that its surface ceases to bear a high 

 enough ratio to its volume, it must starve or divide. 

 Hence the facts of cell -division and the observed 

 limits of cell-size. 



But this is very much less important than an- 

 other result of Spencer's study of the cell in its 

 relation to heredity. We commonly call the cell 

 the unit of life; but this is to ignore the fact that 

 in such cells as those by which individuals of the 

 higher species are reproduced there must be an 

 enormous number of smaller living units. On no 

 other hypothesis can we begin to form any mental 

 picture of the familiar facts of heredity. Spencer's 

 study of the facts led to what Grant Allen, in his 

 monograph on Darwin, calls the "magnificent 

 all-sided conception of physiological units." The 

 vast importance and the amazing adequacy of this 

 conception, reached by the sheer intellectual pow- 

 er of one who was, in the literal sense, merely an 

 amateur of biology, are attested in many ways. 

 An indefinite number of leading biologists have 

 followed in Spencer's track, each reproducing the 

 "physiological unit" 1 in different language but 

 with some lack of its completeness. Imperfect 

 subsequent expressions of it are the micellae of 

 Nageli, the idioplasm of Weismann, the compound 

 organic molecule of Pfluger, the plastidule of 

 Haeckel, and half a score more. For an estimate 



1 The essence of the idea is that of a living unit intermediate 

 between the morphological unit, which is the cell, and the 

 chemical unit, which is the molecule, e.g., of albumin. 

 158 



