42 THE president's address. 



megascleres have disappeared, so that these sponges bear a 

 strong superficial resemblance to the primitive skeletonless 

 Myxospongida — an excellent example of convergent evolution. 



The loss of spicules to which we have been referring cannot 

 be looked upon as an adaptive modification, nor can we explain 

 it as due to mechanical necessities. We know of nothing in the 

 conditions of life to account for it, and can only suppose that 

 it is due to some change in the germ- plasm affecting the 

 power of the sponge to produce the particular spicules in 

 question. 



How can we reconcile these facts with the belief that evolution 

 has taken place, in the main, by slow, successive modifications 

 rather than by sudden mutations ? The conception of factors is 

 intimately bound up with that of mutations, and the existence 

 of the one would seem to imply the occurrence of the other. We 

 must remember, however, that the modifications which seem to 

 be due to separate factors are only rarely and accidentally of 

 value to the organism, so that they hardly ever, in themselves, 

 lead to progressive evolution, and the fact, if it be a fact, that 

 progressive evolution is accompanied by an increase in the 

 number of factors in the germ-plasm is no sort of proof that 

 the factors themselves are of primary importance in determining 

 the course of that evolution. 



If we look at the progressive evolution of the sponges as a whole 

 we see that it has been a gradual process of increase in complexity 

 of structure due to colony formation and integration, in which 

 branching and budding, folding and secondary fusion, have played 

 the chief parts, while the skeleton has constantly become adapted 

 to suit the new mechanical requirements. If, in the course of 

 this evolution, certain categories of spicules have been influenced, 

 even to their disappearance, by certain factors in the germ- 

 plasm, this does not necessarily imply that the evolution of these 

 spicules in the first instance took place by way of mutation. A 

 great cathedral may develop slowly and gradually for centuries 

 under the hands of successive generations of architects and 

 workmen, and in the end it may be destroyed in a few hours 

 in consequence of a sudden outbreak of hostilities. Similarly the 

 loss of characters in living organisms may be due to the appear- 



