THE BIOGENETIC LAW. 339 



of divergent classes and orders, wliile the later forms are 

 more fixed and invariable as the result of heredity and 

 unchangeable conditions of existence; also the principles 

 of morphology and of embryology show that the higher or 

 more specialized animals have descended from more gener- 

 alized forms, and the study of embryology led Von Baer to 

 formulate the so-called biogenetic law, i.e., that the devel- 

 opment of the individual is an epitome of that of the class 

 to which it belongs. 



Another potent factor in species-mahing is geographical 

 isolation, as exemplified by cave animals, desert animals, 

 and those so separated by mountains or differences in tem- 

 perature and other barriers that intercrossing with allied 

 sjoecies is prevented. 



Among the secondary factors are natural and sexual 

 selection, hybridity, but more especially heredit)', and the 

 biological environment, embracing parasitism and the 

 reactions due to the competition and attacks of other forms 

 of life. 



Literature. 



Lamarck's Philosophie Zoologique. 1809. 2 vols. 

 Darwin's Origin of Species. 

 Wallace's Natural Selection, and Darwinism. 

 Semper's Animal Life. 



Romanes' Darwin and after Darwin. 3 vols. 

 Spencer's Factors of Organic Kvolution. 1889. 

 Cope's Factors of Organic Evolution. 1895. 



With the wvitiugs of Haeckel, Hyatt, Weismann, Ryder, Osborne, 

 Dall, LeConte, Jackson, Ilertwig, Lloyd Morgan, and others. 



