THE RELATIONSHIPS OF THE SEA-SPIDERS. 



159 



group before it (the group) diverged into those forms 

 existing to-day. In Fig. 5 such a condition is shown 

 by B. At X in this figure we may suppose, by some 

 means or other, a stage of development (not an ances- 

 tral one), to have appeared, and the young to have 

 become adapted to an independent existence, or to 

 have been fitted by a process of natural selection to 

 lead a free life. Now since the adult form, which 

 developed this secondary larva, subsequently was the 



r 



■ct 



-ay 



x\jz 



ayz 

 xyz xyz 



G 



starting-point for a group of animals, we would expect 

 each species to retain this secondary larval form. Thus 

 in diagram B, Fig. 4, we may suppose a larval form a 

 to appear, and afterwards as the group arose giving x, x, 

 each to have retained the larva a. 



There is possibly a third condition by which a larval 

 form may appear within a group. Briefly put, it is 

 something like this, an adult animal which subsequently 

 gives rise to a group, may have had at its starting-point 



