Chap. IV. TANAIS DUBIUS. 23 



to be fitted to throw light upon a question in which 

 Bronn saw " the first and most material objection 

 against the new theory," namely, how it is possible 

 that from the accumulation in various directions of the 

 smallest variations running out of one another, varieties 

 and species are produced, which stand out from the pri- 

 mary form clearly and sharply like the petiolated leaf 

 of a Dicotyledon, and are not amalgamated with the 

 primary form and with each other like the irregular 

 curled lobes of a foliaceous Lichen. 



Let us suppose that the males of our Tanais, hitherto 

 identical in structure, begin to vary, in all directions as 

 Bronn thinks, for auglit I care. If the species was 

 adapted to its conditions of existence, if the best in this 

 respect had been attained and secured by natural selec- 

 tion, fresh variations affecting the species as a species 

 would be retrogressions, and thus could have no prospect 

 of prevailing. They must rather have disappeared 

 again as they arose, and the lists would remain open to 

 the males under variation, only in respect of their sexual 

 relations. In these they might acquire advantages over 

 their rivals by their being enabled either to seek or to 

 seize the females better. The best smellers would over- 

 come all that were inferior to them in this respect, unless 

 the latter had other advantages, such as more powerful 

 chelae, to oppose to them. The best ^claspers would over- 

 come all less strongly armed champions, unless these 

 opposed to them some other advantage, such as sharper 

 senses. It will be easily understood how in this manner all 

 the intermediate steps less favoured in the development 



