98 SECTIONAL ADDRESSES. 



relative upward movement in tlie direction of the exhalant stream. Here 

 without a doubt Rimula may find its real significance as a connecting link. 

 The stream is horizontal in Emarginula, oblique in Rimula, vertical in 

 Fissurella. The stream would then continue to play upon one point only, 

 in Rimula halfway down the old slit-band, in Fissurella at its earliest base. 

 So playing, it would keep the slit open at the same spot throughout and 

 continue to discharge through the same gap. But the mantle, going on 

 with its general growth, would soon prolong the edges of the slit beyond 

 the range of any inhibiting influence from the cloacal stream. The edges 

 would necessarily meet below it, and would there tend to resume their 

 interrupted continxiity. The mutation I have spoken of would thereby 

 be accomplished, and its adaptive character would need no separate 

 explanation : adaptation itself would have made the hole, and would 

 have simultaneously ceased to make a migratory slit. 



There is an old German proverb which needs to be hung over the 

 mantelpiece of those of us who have a bent for speculation : Behaupten ist 

 nicht beweisen. Nevertheless, if art is long, science is much longer, and 

 of all sciences Zoology makes the greatest drafts on time for securing 

 synthetic results. By ourselves in this field we can do nothing. We must 

 critically assimilate the work of our jiredecessors and co-operate whole- 

 heartedly with our colleagues, or v.e plough the sands. I trust that I have 

 not misused the presidential opportunity and privilege by this mingled 

 play of criticism and suggestion, and that it may help to clarify some of 

 our evolutional problems. Even if every conclusion it expresses should 

 turn out to be untenable, there are times when it is useful to throw the 

 windows of the mind wide open. 



