THEORY OP THE EMBRYONIC PHASE OF ONTOGENY. 327 



bud ontogeny there is no trace of anything which can be inter- 

 preted as ancestral structures, and that some most striking 

 and recent changes, such as the loss of limbs in snakes or the 

 reduction of the toes of the ostricli to two, are not recorded in 

 embryology, i. e. the organ concerned shows from its inception 

 the adult arrangement. 



How then does the theory we have adopted account for the 

 retention of ancestral characters by larvfe ? 



So far as we can judge by comparative anatomy, the stimuli 

 to evolution (in the sense of change of structure) have been 

 two, viz. (1) change of environment and habits, and (3) 

 increased or decreased demands on the working of certain 

 organs. As we therefore pass along a series of genetically 

 connected animals, we should find, pari passu with the 

 environment and the functional demands of the organism, the 

 structure changing. If these stimuli commenced to act from 

 the beginning of free life, then each individual adult in the 

 chain would show from the beginning the modified structure 

 belonging to it; but if these stimuli were deferred in their 

 operation till the animal had attained a certain size, then what 

 was before a uniform life-history would become differentiated 

 into two periods — a larval during which the ancestral habits 

 were retained and the structures corresponding to them, and 

 an adult in which new habits were assumed and structure 

 correspondingly modified. 



An illustration will make this clear : if young flat-fish when 

 they emerge from the egg were at once to adopt the adult 

 mode of life, then that most interesting larval stage, in which 

 they are bilaterally symmetrical, would be missed out in their 

 development. 



Thus we see as a race of animals progressed from point to 

 point in evolution, it would tend to develop atrailoflarval 

 stages, each grade of development surmounted being re- 

 presented by a new larval stage intercalated in the ontogeny. 

 This process, however, could not go on indefinitely ; there 

 would soon arise the tendency for the earlier larval stages to 

 be passed over whilst still in the egg-membrane, and so a 



