HEREDITY 237 



in the development of the characters that are represented by the 

 small differences in the morphologies of the mating parents. For 

 each morphological character (as above defined) there is a gene 

 in the ovum (or spermatozoon) of a parent. If the gene is not 

 there no corresponding character will develop. Yet the develop- 

 ment of such a unit-character requires, not only its corresponding 

 gene, but all the other genes. And, of course, it requires also the 

 " external " factors " of the environment. The cruder Mendelian 

 speculations regard the genes as material particles, thus : "If 

 we magnified a hen's egg to the size of the world (which would 

 make atoms rather larger than eggs and electrons barely visible) 

 we could still get a gene into a room and probably on to a small 

 table " (though the more cautious expressions do not suggest this). 

 We have already seen that it is very improbable that a material- 

 energetic system, of itself, can be regarded as a causal agency in 

 a developmental process. 



Consider the " chromosome-maps " of the fly Drosophila, as 

 drawn by Morgan. All the chromosome material is divided up 

 into the loci of the genes of the Mendelian characters. Thus there 

 is no mechanism (of genes) in the nuclei that accounts for the 

 development of the specifically morphological characters — there 

 is, apparently, no ensemble E. Morgan and his pupils indeed 

 disclaim that their hypotheses involve, in them, a hypothesis of 

 development. Yet it is clear that these hypotheses do involve 

 a hypothesis of development of the loose Mendelian characters or 

 they may frankly ignore the problem of development and merely 

 state how the characters of parents, that differ slightly from each 

 other reappear in the progenies. Genetics may thus be a study 

 only of the reappearances, rearrangements, etc., of the differences 

 of the parents in the offspring. 



82. ON THE ESSENTIALS OF MENDELISM 



(i) Organisms reproduce sexually and all the specific characters 

 of the parents reappear in the progeny. 



(2) But there are always slight differences (that are " inherit- 

 able ") between the parents in respect of their morphologies. 

 Every such slight difference (blue eyes in one parent and brown 

 eyes in the other) is regarded as a " unit-character." 



(3) These differences " go into " the sexual crossing {via the 



