PROBLEM OF DIFFERENTIATION 5 



which principally concerns the science of genetics. The modern 

 tendency is to accept the principle of a germ-plasm while recog- 

 nising that it is not as inaccessible to the modifying action of ex- 

 ternal factors as Weismann contended. The question oi the function 

 of the particles or factors in converting the fertilised egg into the 

 body of the adult is the concern of that modern and rather special 

 branch of embryology usually called physiological genetics. 



Before dealing with the conclusion derived directly from experi- 

 mental work, a moment's attention may be turned to philosophical 

 criticisms of the preformationist view that particles, determinants, 

 or any hereditarily transmitted units or factors, can ''explain" 

 development. First of all, Aristotle pointed out that certain features 

 in which offspring resembled parent could not be ascribed to 

 the transmission of particles from corresponding parts, for the 

 latter might be dead structures like nails or hair from which no 

 particles could be expected to come, or again they might be such 

 characters as timbre of voice or method of gait. He goes on to say, 

 by way of illustration, that if a son resembles his father, the shoes 

 he wears will be like his father's shoes, yet there can, of course, be 

 no question of particles here. In other cases, resemblance may refer 

 to structure, plan or configuration rather than to the material of 

 which it is composed, and it is hard to see how particles can repre- 

 sent such structure, plan or configuration. Again, how is the 

 eventual beard of a son to be explained if he was born to a beardless 

 father? To these objections might be added the insuperable diffi- 

 culty of accounting for the production of oflrspring structurally 

 different from the parent, as when the egg laid by a queen bee 

 develops into a worker, or, even more generally, when a mother 

 bears a son or a man fathers a daughter. 



If, then, particles coming from corresponding parts are not re- 

 quired in some cases and cannot be resorted to in others in order 

 to explain development and hereditary resemblance, why should 

 they be postulated in any case? This, of course, concerns genetics 

 as much as embryology, but Aristotle came very close to the crucial 

 problem of the latter when he wrote : "either all the parts, as heart, 

 lung, liver, eye, and all the rest, come into being together, or in 

 succession.... That the former is not the fact is plain even to the 

 senses, for some of the parts are clearly visible as already existing 



