PRECIS OF CONTENTS xi 



The establishment of permanent isolation. Analysis of the various methods 

 of establishment. Discussion of (a) Seasonal occurrence ; (b) Time 

 of breeding ; (c) Loss of means of dispersal, etc. (indirect methods) ; 

 and (d) various bars to intercourse, and (e) various sources of 

 sterility (direct methods). Conclusions : Importance of biological 

 races. 



CHAPTER VI 



CORRELATION 



Use of the term ' correlation ' ; Diirken's analysis. ' Physiological ' and 

 * gametic ' correlation (Graham Kerr). The correlation of specific 

 characters very variable in degree, probably on the average rather 

 low. The two fundamental types of correlation (' causal ' and ' coin- 

 cidental '), their causes and importance. Methods of deciding the 

 basis of the correlation of specific characters. Segregation and specific 

 characters in relation to correlation and variation. Highly correlated 

 characters are probably those for which large populations are homo- 

 zygous. ' Lineages ' and the correlation of specific characters. 

 Independence of characters as revealed by the study of ' lineages.' 

 Difficulty of reconciling the apparent independence of characters in 

 phylogeny with our conception of development and organisation. 

 Specific characters as mosaics of fortuitously associated units. Their 

 incorporation in the general unity of the organism and transformation 

 of their basis from a fortuitous to a permanent one. 



CHAPTER VII 



NATURAL SELECTION 



Introduction. Darwin's presentation of the evidence. Subsequent 

 modification and development of the theory. Conditions of proof 

 required and procedure to be adopted in this work. The origin of 

 domesticated races and its relevance to the problem of Evolution. 

 Selection experiments with pure bred stock. Pearl's requirements of 

 proof that selection has altered the character of a race. 



Direct evidence for the selective incidence of death-rates in nature. Twenty 

 cases of direct observation on the selective incidence of death-rates 

 examined. Summary of the examination (p. 212). Direct observa- 

 tion on the alteration of natural populations. Summary (p. 215). 



The nature of variation considered in relation to natural selection. The 

 mutation-rate and survival-value of mutations. Mathematical treat- 

 ment of the subject. The problem of random mating. Summary 

 (p. 229). 



Indirect evidence for and against the efficacy of selection. 



I. Standard cases. Protective coloration. Mimicry. 

 II. Less intensively studied cases. The adaptations of torrent-living 

 animals. The colour and pattern of Cuckoo's eggs. The adaptations 

 of (a) abyssal animals and (b) cave-dwelling animals. 



