OPENING ADDRESS. 23 
probably act as sensory organs, and they form a more or 
less perfect grid for preventing large objects from entering 
the branchial sac. Hence there can be no doubt that in 
this case also the various modifications are really useful. 
I feel that I am still only on the threshold of this 
inquiry, but these few instances are perhaps sufficient to 
show why it is that I believe that specific and generic and 
other diagnostic characters are of actual importance to 
their possessors, are in fact such adaptive modifications as 
would be produced by the action of natural selection; and 
therefore I have no hesitation in recommending this 
subject to you as one of the most interesting and fruitful 
lines of investigation in the whole field of marine Biology. 
