3o6 



ESSENTIALS OF BIOLOGY 



or behaviour, for very long periods of time. As a rule, the same 

 natural region will be populated by many kinds of animals and 

 plants, but this does not matter in the present exposition. We 

 may represent the succession of generations of the stock, year 

 after year, by an unbroken straight line, or band (i. Fig. 38) ; 

 the thickness of the band indicating the density of the population; 

 its length indicating the period of time during which the popula- 

 tion has existed and the continuity of the line, or band, suggesting 

 that there has been no change in the characters of the individual 

 organisms forming the species : 



■^^ Time 



TuTw, FamUxf A 



Fig. 38, 



Now let a number of mutations occur so that, in a certain 

 fraction of the population, the characters of the species change, 

 a new sub-species, or variety, or race, coming into existence. We 

 represent such a transformist process as in 2, Fig. 38. Next we 

 may imagine that the transformist process continues, so that 

 mutations appear among the individuals of the new race, and 

 mutations among these latter kinds of individuals, and so on. 

 We may represent such occurrences as in 3, Fig. 38, where 

 A represents the original species, which continues to maintain 

 itself unchanged. Just as the line B represents a new race originat- 

 ing in the first mutating individuals of A, so C represents another 

 race originating in mutating individuals of B, and so on. The 

 marking C, will indicate that the species, or race, C, dies out, 

 or becomes extinct. Obviously at each branching the charac- 

 ters of the diverging species become more and more different 

 from those of the original one, A. We may say that a group of 

 species, G, H and I come into existence and these differ so much 



