I46 THE EVOLUTION OF LIVING BEINGS. 



as the one possessed by the common ancestor of those 

 organisms, and that consequently we have no reason 

 to suppose that each of the parts of the different orga- 

 nisms had already a position in that ancestral organism, 

 through which we can spot the homologies of a given 

 part of an organism, now existing. 



It remains perfectly true however, that the members 

 of not too large a group resemble each other in their 

 general plan of organization. 



Now this is stating a fact of the same kind as when 

 we say that the different forms resulting from a cross 

 between two Linneons resemble each other in the ge- 

 neral groundplan of their organisation. The cause 

 must lie — in the latter case at least, in the former we 

 do not even know whether the members of the group 

 belong genetically together — in the constitution of the 

 gametes which united to form the hybrid, which initi- 

 ated the group of new species, and as long as we know 

 nothing of the constitution of these gametes, we can 

 explain, as little how this comes about, as we can explain 

 why one chemical substance cristallizes in the one, 

 another in another form. 



Now one hears it often said, that there is a basic dif- 

 ference between the structure of new Linneons, arising 

 from the cross of two preexisting ones, and between the 

 structure of a new class of organisms, supposedly also 

 arisen from the cross of two individuals — be it from 

 two more different ones — in as much as the general 

 plan of the new Linneons is the same as that of the 

 Linneons crossed, while the general plan of the new 

 class must, on the contrary, be different from that of 



