INHERITANCE OF ACQUIRED CHARACTERS 205 



related in descent have inherited common methods of develop- 

 ment. Thus to embryology we have looked for the final test 

 as to the real affinities of any given form. But even this test is 

 sometimes delusive, for selection and environmental influences 

 may affect embryonic development, as they may affect every 

 other character or instinct. 



Certain writers carry this thought further, and find no real 

 basis for discrimination between homologies and analogies. 

 They would hold that the progressive inheritance of effects of 

 similar environment might in time produce forms not imme- 

 diately related into a condition of practical identity each with 

 the other. 



Professor Hans Gadow observes : 



" When Gegenbaur had become the founder of modern comparative 

 anatomy by putting it on the basis of evolution, it became gradually 

 an axiom that homologies determined the degrees of affinity, and now 

 in turn the position of an animal in the system is appealed to for de- 

 termining whether a given organ is homologous or only analogous. 

 It is a vicious circle. 'Only analogous' is the usual expression. In 

 reality these cases of analogy, homoplasy, convergence have become 

 of supreme interest in our science. Their solution implies the greatest 

 of problems, and it is only the thoughtless orthodox fanatic who be- 

 lieves that similarity in structure must mean relationship. To him 

 two and two make four, no matter what the twos are composed of." 



But most naturalists believe that homology, which involves 

 common descent, and analogy, which rests on similar experi- 

 ences, are quite distinct elements, and that they can always 

 be distinguished by recognized biological tests. 



No one can question the vast influence of extrinsic or en- 

 vironmental influences on the history of a species of animal or 

 plant. In the analysis of such influences we find a wide variety 

 of opinions. According to some writers, these forces are dy- 

 namic, shaping the development of the individual, and by 

 heredity determining through the individual the future of the 

 species. Dall uses these striking words: 



"The environment stands in relation to the individual as the ham- 

 mer and anvil to the. blacksmith's hot iron. The organism suffers 

 during its entire existence a continuous series of mechanical impacts, 

 none the less real because invisible.' 



