390 ELEMENTS OF BIOLOGY 



as the basis of evolution: (i) Variations occur among plants and 

 animals as a result o£ needs or of decrease in the usefulness of an 

 organ; (2) these acquired characters are transmitted to succeeding 

 generations. In a previous chapter (p. 339) the improbability of the 

 inheritance of such modifications was shown; but this doctrine of 

 use and disuse as an influence in heredity, this belief that the char- 

 acters acquired by the adult in response to its contacts with the 

 environment are transmitted to the offspring is in some quarters 

 still vigorously debated. 



Darwin. Lamarck's theory is based on two series of facts that 

 are evident to everyone, namely, that variations occur, and secondly, 

 that offspring resemble parents, in other words, the fact of inher- 

 itance. The feature of Lamarck's conception that is open to question 

 is that variations which occur as a result of somatic changes in the 

 organism (individual adaptations or acquired characters) become a 

 part of inheritance. The next fully formed theory of evolution to- 

 appear is that of Charles Darwin in 1859. Darwin's theory is based 

 on three sets of facts: (i) the occurrence of variations; (2) the fact 

 of inheritance; (3) the fact of natural selection. The first two are 

 of course involved in the formulation of Lamarck's theory. The 

 third, natural selection, required further comment. 



The meaning of this term will be made clear by illustrations. In 

 breeding stock or fowls an artificial selection is practiced by the 

 breeder; those animals which exhibit the desired character are thus 

 artificially selected for mating and the others disposed of. A selective 

 and eliminating principle also operates under natural conditions. 

 Herbert Spencer has called this principle the "survival of the fit- 

 test." Thus if a mutation (p. 341) occurs which unfits the animal for 

 its environment, it and its progeny are soon stamped out. If, on the 

 other hand, the mutation renders the organism better fitted to cope 

 with its surroundings, those individuals having this character soon 

 assume dominance. For example, we may assume the sharp vision 

 of the gull to be derived by evolution. By natural selection sharp- 



