558 



The Earth and Its Inhabitants Change unit x 



Fig. 507 Comitless thoitsands of seeds fell in this daisy field. In which ways may 

 these daisies have been better fitted for survival than those that failed to grow? 

 What kind of inheritable variations might survive here? What kind might be elimi- 

 nated? (AMERICAN MUSEUM OF NATURAL HISTORY) 



caused by the environment or by con- 

 stant recombinations of genes or by 

 gene and chromosomal changes that are 

 passed on to the next generation, Dar- 

 win knew nothing about chromosomes 

 or genes but he did know that no two 

 individuals are exactly ahke. 



4. Survival of the fittest. Since there 

 are always variations among the mem- 

 bers of a species, Darwin argued, some 

 organisms will be more fit to meet exist- 

 ing conditions, some less fit. In general, 

 those organisms that are more fit will 

 survive, the others will die. Darwin 

 pointed out that this would hold true 

 particularly when conditions changed 

 or emergencies arose. In general, the 

 survivors tend to be those organisms that 

 are better fitted than others to meet new 

 conditions. This is what Darwin meant 

 by the survival of the fittest. It is clear 

 that the word fittest does not mean the 

 strongest. It means the organism that 

 fits into the whole environment best, 



whatever the environment may be at 

 that particular time. 



5. hiheritance of variations. This is 

 the last step of Darwin's theory of natu- 

 ral selection. Evidently, the variation 

 which happens to make one organism 

 more fit than its competitors must be 

 passed on to its off^spring if a new spe- 

 cies is to arise. The horse could not 

 have evolved from Eohippus through 

 natural selection unless there were iii- 

 heritance of variatio?is. Natural selection 

 can account for the origin of new spe- 

 cies only if the variation that made an 

 individual more fit was one that could 

 be passed on to the offspring. 



Darwin lived before there was much 

 scientific understanding of heredity. He 

 was therefore unable to explain clearly 

 why some variations are inherited and 

 others not. He was aware of the weak- 

 nesses in his own theory and stated them 

 frankly so that others might look for a 

 more complete explanation. 



