1098 LIFE : OUTLINES OF GENERAL BIOLOGY 



ovum. A new variation must be consistent with what has gone before; 

 bizarre additions to the germinal architecture are very improbable, 

 though it sometimes happens in experimental breeding that there 

 is an occurrence of non- viable or lethal combinations of hereditary 

 factors. In Wild Nature the occurrence of pathological variations 

 seems to be exceedingly rare. There is some fortuitousness in the 

 process by which one member of a pair of homologous chromosomes 

 is given off in the first polar body of a maturing ovum, or in what 

 decides which of two kinds of sperms enters the egg ; but the small- 

 ness of the fortuity here may be inferred from the fact that it is in 

 the reducing or meiotic division that the phenomena of Mendelian 

 Inheritance find their explanation. The fact that the number of 

 chromosomes in a series of related species sometimes forms a numeri- 

 cal series, like 7, 14, 28, 56 in roses, strongly counters the mistaken 

 impression of fortuitous. And if the raw material of evolution is but 

 slightly fortuitous, the same may be said of the selection mill, for 

 it has its foundations in a long-estabhshed, subtly inter-related 

 systemaNaturce. The struggle for existence may discriminate between 

 a Shibboleth and a Sibboleth variation, but its operation is anything 

 but fortuitous. 



14 An important general fact is expressed in the phrase "correla- 

 tion of variations" — an idea which Darwin emphasised. Certain 

 new departures hang together; they may be diverse expressions of 

 one mutant gene, or of a new gene, or of certain genes which always 

 occur close together in a group. Just as an inborn disease may have 

 many different expressions in different parts of the body, so it may 

 be with an integrative germinal variation. This may sometimes be 

 the case when a novelty is still in its initial stages and not large 

 enough to be caught in the selection-sieve; yet it may be selected 

 because it is associated with an older and larger variation that has 

 become well established. 



15. Heredity may be defined as the flesh-and-blood relation of 

 genetic continuity between successive generations, such that a 

 specific organisation is continued on, "like tending to beget Hke". 



16. The inheritance is aU that the organism is or has to start with 

 in virtue of its hereditary relation, and the vehicle of the hereditary 

 initiatives (called "factors", "determinants", "genes", etc.) is 

 mainly[(most biologists say wholly) in the chromosomes of the nuclei 

 of the germ-cells. It is stiU premature to say that part of the inherit- 

 ance may not be carried by the extra-nuclear protoplasm of the 

 germ-cells. The genes are not visible, yet they can be indirectly 

 measured and mapped! They seem to lie in a hnear order along 

 each chromosome. 



' 17 As a chromosome is in life a very fluid structure demarcated 

 by a film (though^it may appear compact and well defined after 

 fixing and staining), we must not think of the initiatives or genes as 



