The Outline of Science 



not give any formula for the causes of the novelties he observed. 

 No doubt he also meant that the organism in varying was not 

 aiming at anything. And yet he laid great stress on what he 

 called "the principle of correlated variability" an idea of great 

 importance that when one part varies other parts vary with 

 it, "being members one of another" as St. Paul said. In other 

 words, a particular germinal change may have a number of dif- 

 ferent outcrops or expressions. But the more correlation there 

 is, the less reasonable will it be to speak of fortuitousness. And 

 one of the changes since Darwin's day is the recognition that 

 variations are often very definite just as they are among 

 crystals. 



6. Another change from Darwin is the Mendelian idea 

 of unit-characters, which behave like entities in inheritance. They 

 are handed on with a strong measure of intactness to a certain 

 proportion of the offspring. Their "factors" in the germ-cells 

 are either there or not there. Sometimes, at least, these unit- 

 characters arise as mutations, and thus we have an answer to 

 Darwin's difficulty that abrupt changes would be averaged off 

 in intercrossing. Unit-characters do not blend. 



7. Since Darwin's day there has been, in a few cases, definite 

 proof of natural selection at work; the different forms of selec- 

 tion have been more clearly disentangled; the subtlety of Dar- 

 win's idea of selection has been confirmed; the reality and the 

 efficacy of preferential mating has been much criticised, but 

 Darwin's theory of sexual selection has in its essentials weathered 

 the storm. In proportion as new departures come about suddenly 

 by brusque mutation, the burden to be laid on the shoulders of 

 selection will be lessened. In so far as the selection is in rela- 

 tion to a previously established system of inter-relations, there 

 will be a reduction of the fortuitous in the process; and the same 

 will be true in proportion to the degree in which the organism 

 takes an active share in its own evolution as it often does. 



8. Modern biologists are inclined to put more emphasis on 



