CONTENTS xiii 



XIX. The Physical Basis of Inheritance 289 



The Sulton-Boveri hypothesis; sex determination and sex linkage; 

 limitations to Mendel's second law; linkage, crossing-over, and 

 the linear order of the genes. 



XX. Some Complications of Mendelian Inheritance 302 



Absence of dominance; lethal characters; multiple effects of a single 

 gene; genotype and phenotype; the interaction of genes; the multiple- 

 factor hypothesis. 



XXI. Variation as the Basis for Hereditary Distinctions. . . . 311 

 Environmentally produced and autogenous variations ; variations due 

 to chromosomal and to gene changes; multiple allelomorphs; hybridi- 

 zation and new combinations of genes; heredity and environment. 



XXII. Inheritance in Man 318 



Heredity versus environment; the eugenics movement. 



PART III. THE CHANGING GENERATIONS: THE EVOLUTION OF LIFE IN 

 TIME AND SPACE 



XXIII. The Evolutionary Concept 329 



The variety and multiplicity of living things; early evolutionary 

 thought; Darwin and the establishment of organic evolution as a fact; 

 Darwin's explanation of the causes of evolution; subsequent modifi- 

 cations of Darwinian theory; variation as the "raw material" of 

 evolution; inheritance and the mechanisms of gene assortment; 

 selection; isolation; population dynamics. 



XXIV. Some Consequences of Evolutionary Relationship .... 354 



The blood relationships of organisms; homology as an expression of 

 relationship; taxonomy as an expression of relationship; adaptive 

 radiation and adaptive convergence; biogeography and evolution. 



XXV. The Geological Background of Evolution 387 



The earth as the evolutionary stage; rocks and strata; the history of 

 the continents; geologic time and evolution; fossils and their 

 significance. 



XXVI. The History of Plants t 399 



The beginnings of life; thallophytes, the earliest plants; ancient land 

 plants; the rise of modern floras. 



XXVII. Ancient Animals 419 



The Cambrian fauna; the early vertebrates; the Devonian fishes; 

 the rise of amphibians and insects. 



XXVIII. Reptiles, Birds, and Mammals 437 



The origin of reptiles and their Mesozoic radiation; the history of 

 birds; the history of mammals; the evolution of the horses. 



