46 THE president's address. 



another. If all cars of the same kind were painted of the same 

 colour the analogy would of course be more complete. 



In conclusion, if any one should ask me how I should propose 

 to define the term " species " as applied to sponges, I could only 

 reply that, at any rate in the present state of our knowledge, no 

 satisfactory definition is possible. In practice we give the name 

 to any group of individuals that closely resemble one another 

 and are at the same time distinguishable from any other 

 group by one or more characters that they possess in common. 

 Such groups may have arisen in divers ways, as by sudden 

 mutation or by the dying out of connecting links in the course of 

 gradual divergent evolution, and it is quite possible that 

 hybridisation has also played a part. It is more than probable 

 that many groups now regarded as distinct species will ulti- 

 mately be found to be connected by more or less perfectly 

 graduated series of intermediate forms. This probability, how- 

 ever, has no terrors for the philosophical systematist, for one of 

 his principal objects is to reconstruct the phylogenetic tree and 

 demonstrate the genetic relationships of all the members of the 

 group on which he works, and from this point of view the more 

 connecting links he discovers the better he will be pleased. 

 Whether his species are good, bad, or indifferent, is a matter of 

 secondary importance, though of course every reasonable worker 

 will avoid as far as possible the unnecessary multiplication of 

 specific names. 



Journ. Quekett Microscopical Club, Ser. 2, Vol. XIII., No. 78, April 19l6i 



