190 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



we define the law of acceleration of development to mean that 

 in a progressive series the young of the descendants corre- 

 spond to the adults of their more remote ancestors, we find 

 that this does not apply to a retrogressive (retarded) series. 

 In this latter case we must restate the law as follows : The 

 adults of descendants correspond to the young of their more 

 remote ancestors, the higher generic stages to which these 

 ancestors attained having been dropped away by successive 

 retardation, or arrested development. The retarded series 

 themselves may become the radicles of new stocks, and so 

 we may have cases where the ontogeny of any one species 

 or genus can never give the full history of the race. 



Groups Available for Cori'elation. — We see, then, that 

 the student of morphogeny of animals has to be on his 

 guard, first against the loss of generic stages during the 

 period while the animal is in the Q^^g\ then against the 

 introduction of secondary larval stages when the ancestors 

 lacked them; then against the introduction of secondary 

 characters due to adaptation ; then against unequal acceler- 

 ation, bringing together, in the ontogeny of the descendant, 

 characters that occurred in separate generations of ances- 

 tors; and lastly, against retardation, by which the form 

 never reaches the full generic evolution of its ancestors, 

 and where, if a new series starts out from the retarded 

 form, the complete family history is not recorded in 

 ontogeny. 



Is it to be wondered at, then, that the student of mor- 

 phology becomes a sceptic, or even a rank unbeliever with 

 regard to the value of ontogenic stages as records of 

 history ? It is only to be expected that the biologist, espe- 

 cially one that deals almost exclusively with living species, 

 should be inclined to discredit the law of tachygenesis, and 

 to believe that there is such an inextricable muddle of omis- 

 sions, secondarily introduced characters, and unequal accel- 

 eration of those actually repeated, that the record is wholly 

 untrustworthy, or at least illegible. And yet there are so 

 many species and genera in the various groups of inverte- 

 brates whose ontogeny is simple, progressive and fairly 



