The PRTxcirLES of Palaeontology. 177 



panied b.v con siMi^rable modification in the general shape of the 

 body, lie li;is l«>llo\ved out the modifications in diverse types 

 and (Inn. uistiatcd that these temporary stages were attained in 

 a p«M'm;«neiiL state in extinct forms which must be considered as 

 ance>tral. 



The ('inbr3^ogenic study of fossil forms is still in its infancy, 

 ard ]);'osents always greater difficulties than does the study of 

 living organisms. It is impossible, indeed, to take a direct view 

 of the development of the embryo, and the organism seldom 

 bears traces of the forms it has passed through in its embr3'onic 

 state. We are obliged, then, to content ourselves with carefully 

 comparing the forms which are considered as successive phases of 

 the development. Despite these obstacles the path is so fertile 

 in results that palaeontologists do not hesitate to enter on it ; we 

 shall see, even in the Yertebrates, that many types described as 

 separate species, are now considered to be embryonic forms of 

 organisms which are found in the adult state in the same layers, 

 and that this discovery has furnished valuable conclusions for 

 phylogenetic data. 



Cases of regression. — The law of the parallelism of Ontogeny 

 and Phylogeny makes possible, th-en, the elucidation of many 

 questions left undecided by Comparative Anatomy. This law, 

 indeed, is able to indicate the right explanation of variations 

 which may admit of various interpretations ; for instance, we 

 are often perplexed in comparing various forms of unequal grade 

 of organization to decide whether their filiation marks progress 

 or, on the contrary, regression ; in the latter case the form the 

 most simple in appearance may bear traces in the embryonic 

 state of primitive complication ; we have just shown this to be 

 so in regard to certain Ammonites. 



The same remark applies also to the evolution of each organic 

 system individually considered. The embryogenic development 

 displays in many cases organs which at first develop according to 

 the rules normal • for the group under consideration, then the 

 development is arrested, and the organ retrogrades. Thus are 

 formed the rudimentary organs of which we have spoken in the 

 preceding chapter. The existence of these organs in no way 

 implies an inferior rank for the animal itself, but it may lead to 

 specialization, to the adaptation to some particular mode of life. 

 23 



