THE CONTINUITY OF LIFE 15 



strength of the muscles of the legs, but has effected little in 

 the way of actual change of structure, so that anatomically 

 man still stands very near his arboreal kinsmen that represent 

 the immediate past in the history of human development. 



This study of the succession of forms upon the earth is 

 termed race history or phylogenesis, and forms one of the two 

 sources from which the past history of ' animal development 

 may be obtained. The other is the sequence of stages re- 

 corded during the embryonic development of each individual, 

 and is termed the developmental history or ontogenesis. By 

 what is at once the most natural and the most mysterious law 

 of nature each individual animal inherits, not only the struc- 

 ture of its immediate parents, the attainment of which means 

 the end of its development, but also that of its entire line of 

 ancestors, which appear in approximately the natural order 

 of succession and constitute the stages of its ontogenetic de- 

 velopment. 



As a result of this it follows that the two records, phylo- 

 genetic and ontogenetic, run closely parallel, and each serves 

 in many places to bridge a gap or explain an obscure period 

 in the other. This parallelism of the two records lies at the 

 basis of all morphological speculation, and forms what is 

 often termed the law of biogenesis* It must not be expected, 

 however, that the correspondence in the two records is com- 

 plete, since numerous disturbing causes must be taken into 

 consideration which tend to modify each record quite inde- 

 pendently of the other. In the race history there are many 

 gaps caused by extinction, and the forms that have come 

 down to us from earlier periods have become much changed 

 from their former condition and represent their ancestors in 

 a qualified sense only; while in the individual development 

 there are many characters that are in no sense historic, and 

 have to do with such immediate environmental problems as 

 nutrition or protection. These latter characteristics, which 



* The " Biogenetisches Grundgesetz " of Haeckel ; formulated by him 

 as follows: "Die Ontogenie (Keimesgeschichte) ist eine kurze Wieder- 

 holung der Phylogenie (Stammesgeschichte)." 



