[Chap. XL VARIATIONS AND DIVERSITY OF ORGANISMS 497 



billion years or more, and they are the fundamental underlying steps of 

 the great diversity of plants and animals on the earth today. Geologically, 

 the most recent kind of plants are the seed plants, and the most recent 

 animals are the mammals. Since they were formed by changes in a 

 comparatively few of the hereditary units of their ancestors, we may 

 expect to find them fundamentally much more like than unlike their 

 ancestors. In fact, we may expect to find in them certain basic features 

 that were in their remote ancestors many hundreds of millions of years 

 ago. These basic features are, of course, the fundamental physiological 

 processes that are essential to all living organisms. Variations in heredi- 

 tary factors that inhibit, or unbalance, any one of these fundamental 

 processes are lethal, and they result in the extinction of organisms when 

 not annulled by alternative dominant factors. 



The initial steps in the formation of a new species, then, may be 

 visualized somewhat as follows: origin of heritable differences by such 

 changes in chromosomes and genes as those described in the preceding 

 chapter; segregation of these differences in certain individuals; a de- 

 crease in cross-fertility, or a decrease in cross-fertihzation because of 

 geographic isolation; and an increase in homozygosity and distinct in- 

 dividuality. 



The descriptive method of approach to the study of evolution was the 

 one followed by our forefathers before chromosomes and Mendel's law 

 of hybrid segregation were knovm. This method includes the study of 

 the visible characters of organisms; their methods of development and 

 reproduction; and their fundamental similarities, differences, and rela- 

 tionships, including those of fossils. Some of these facts will be evident 

 in subsequent chapters. 



The study of evolution from this point of view led to the discovery of 

 several fundamental principles about living organisms. A few of those 

 upon which there is general agreement are briefly stated below. They 

 will be of greater interest if each one is considered with reference ( 1 ) to 

 the probability of its being explained on the basis of the facts you know 

 about chromosomes and genes, and ( 2 ) to the extent to which any pre- 

 conceived ideas you have held about the subject are incorrect. 



The kinships of organism, both living and fossil, are interpretable only 

 on the basis of descent by modification (evolution) through a long period 

 of time. 



Diversity of organisms is the result of evolution. That is, species are 

 not specially created, unchangeable units. 



