The Principles of Palaeontology. 179 



which should faithfully reproduce the phylogenic evolution, is 

 modified by the intervention of a new force distinct from 

 heredity. 



It is evidently for the advantage of the species that the embry- 

 ogenic development should take place as rapidly as possible, since 

 during the embryonic stages the young individual is more 

 exposed than in the adult state. The law of embryogenic accel- 

 eration is then a consequence of natural selection. It consists in 

 this, that the highest forms in each group develop with more and 

 more rapidity ; the stages corresponding to ancestral forms nota- 

 bly differing from the definitive form, may in some cases be 

 skipped. This occurs more especially in the first stages of develop- 

 ment. In species, even very closely allied these stages present 

 such a diyersity that we are often unable to utilize them in seek- 

 ing for remote ancestral forms. Accessory circumstances, such 

 as the greater or less quantity of accumulated nutritive materials, 

 or the appearance of protecting membranes, or the structure of 

 the temporary larval organs, allowing transient adaptation to the 

 medium where this period of development is passed, alter the 

 normal succession of the phases, and conceal the normal 

 embryogeny. 



In Palaeontology, embryogenic acceleration has been espec- 

 ially proved among the Ammonites, for the embryonic stages are 

 so preserved that the successive volutions represent the different 

 stages through which the animal has passed. The characters of 

 a given form will be reproduced, in the development of the 

 descendants of this form, in stages more and more precocious, and 

 may end by never appearing again. We may notice the argu- 

 ment which Hyatt has drawn from these facts for the filiation of 

 the Arietidae. In this class of Ammonites the septum of the 

 primary chamber is seen to resemble exactly that of the lower 

 forms of Nautilus ; in the higher forms this first septum presents 

 a slight curve which, in the immediate ancestors, appears only in 

 the second septum ; finally, higher still, to this first curve now 

 become more accentuated, there is added on each side a lateral 

 angulation which reproduces the third septum of the ancestral 

 forms (Branco). 



The embryogeny of the Trilobites, studied by Barrande, shows 

 that the acceleration in this order attains a very variable degree 



