74 BROOKS— HEREDITY AND VARIATION; [April 20 



that all men are particular men, concrete and unique ; that the normal 

 man is a fictitious character, a statistical average, reached by ignor- 

 ing all that is distinctive of each human being. 



One can easily see why the notion that species is in germ cells 

 has come to prevail. Nothing in nature, except the human mind, 

 is easier to contemplate as an independent, self-sustaining, self- 

 sufficient whole than is an egg. The symbolical comparison of the 

 universe to an egg appeals to all, for nothing is easier than to 

 think of an egg as a metaphysical thing in itself, a self-centred and 

 self-sufficient microcosm. For many of the practical purposes of 

 the scientific embryologist it is convenient and legitimate to regard 

 it as a compete and self-sufficient being, but one must not forget 

 what these practical purposes are, for the use of a concept for a 

 practical purpose is apt to end in belief that it is true in general and 

 useful for all purposes, and thus to entangle one in unforeseen para- 

 doxes. 



Every reflective biologist must know that no living being is self- 

 sufficient, or would be what it is, or be at all, if it were not part of 

 the natural world, although no truth is easier to lose sight of. Liv- 

 ing things are real things, and we can never know too much about 

 them, but their reality is in their interrelations with the rest of 

 nature, and not in themselves. 



Surely, this is good sense and good science. No physiologist 

 who studies the waste and repair of living bodies ; no naturalist who 

 knows living beings in their homes ; no experimental embryologist 

 who studies the influence of conditions, internal and external, upon 

 development, should, for an instant, admit that a living being is 

 self-sustaining or self-sufficient, or that its being is in itself ; for the 

 line we draw, for our own convenience, between living things and 

 the external world, is not one that we find in nature, but one that 

 we make for our own purposes. 



The external world of a living being is as essential to it as its 

 histological structure. If the environment of its body, or of any 

 cell within its body had been different, neither cell nor body would 

 be what it is, and if they had no environment they would not be 

 at all, for neither seeds nor eggs nor desiccated rotifers exist ab- 



