58 BIOLOGICAL LECTURES. 



both European and American. Wiirtenberger points out that 

 the so-called Ammonites mutabilis is not a true species, but a 

 composite group, made up by the convergence of several dis- 

 tinct lines to a common. This case is peculiarly significant, 

 because it would hardly have been detected had not the 

 embryonic and young stages of the shells been preserved. 



It seems the most obvious of commonplaces to say that 

 numerous and close resemblances of structure are prima facie 

 evidences of relationship. Yet the statement is true, even 

 though the resemblances have been independently acquired, 

 because parallelism is a more frequently observed phenomenon 

 than convergence, and because the more nearly related any 

 two organisms are, the more likely are they to undergo similar 

 modifications. 



All this brings us back to the thesis so frequently insisted 

 upon already, that the only safe and trustworthy method of 

 constructing phylogenies is by tracing the development, step 

 by step, through all its gradations ; and until this is done the 

 classification of any group can be but tentative and provisional, 

 that is, if we intend classification to express relationship. 



No department of biological science is at present the scene 

 of such vigorous controversy as that which deals with the 

 factors of evolution, the causes which determine the develop- 

 ment of new forms, and the problems of heredity which are 

 inseparably connected with them. Palaeontological evidence 

 will prove to be of much importance in this connection also, 

 but it cannot well have more than a corroborative value. 

 Though the examination of long and complete phyla brings to 

 light much that is suggestive concerning the factors which 

 have brought these changes to pass, and any rational theory 

 must embrace and explain these facts, yet the deciding weight 

 must probably come through the physiological and experi- 

 mental method. Time fails to deal with such far-reaching 

 questions here, and yet it may be well to call attention to the 

 necessity of avoiding a dogmatic and intolerant attitude, and 

 to deprecate any premature attempt to exclude this or that 

 class of factors from consideration. In most of the recent 

 writings upon the efficient causes of evolution you will find 



