DARWINISM DEFENDED. 167 



hygienic conditions, yes, even in temperament, determine 

 whether there shall be healing or death ? The iridescent 

 colours of many male birds, butterflies, and certain parasitic 

 copepods (Sappharitta) , are certainly of a nature to pro- 

 duce a great effect on the eye, but these colour effects are 

 not the result of special pigments but of microscopically 

 minute structural conditions. In Africa the tsetse-fly 4 ex- 

 tends over large regions, and only those cattle with a skin of 

 a certain thickness, so that the tiny proboscis of the fly 

 cannot penetrate it, can live in these regions immune from 

 the fatal attacks of the pest. Many poisons work in almost 

 infinitely weak dilution a 1-200,000 solution of ricin, for 

 example, is able to kill mice." 



Plate presents a second type of answer to the objection 

 by calling on certain aids or auxiliary principles by whose in- 

 fluence a difference at first unimportantly small 



Correlation 



may aid slight gradually comes to be transformed into one of 

 selective value, or may reach this stage sud- 

 denly by means of a change in life habits. This may come 

 about "through correlation, 3 that is, through that unknown 

 law of growth by which an indifferent organ may be so 

 bound up with or related to a useful organ 8 that it, the 

 indifferent organ, is perfected along with the useful organ 

 as this latter is developed or specialised through selection. 

 All organs of an animal are intimately related to and influ- 

 enced by one another: each is in relation to the other just 

 as to the outer world. How close this inter-dependence is, 

 is most easily appreciated by one in his own case when sick : 

 a constipation causes headache, a slight diarrhoea affects the 

 composition of the urine, etc. The correlation can be so 

 intimate and important, as the case of the secondary sexual 

 characters shows, that its origin and development depends 

 directly on particular definite stages of the related organs." 

 In the case of many animals the appearance of various curi- 

 ous and large modifications of legs, wings, skin, feathers, 



