10 
THE SILURIAN BEACH. 
the fundamental types, while the species repre- 
senting these types have differed in every geologi- 
cal period. Now what we call typical features 
of structure are in themselves no more stable or 
permanent than specific features. If physical 
causes, such as light, heat, moisture, food, habits 
of life, etc., acting upon individuals, have gradu- 
ally in successive generations changed the char- 
acter of tlio species to which they belong, why 
not that of the class and the branch also ? If we 
judge this question from the material side at all, 
we must, in order to judge it fairly, look at it 
wholly from that point of view. If these specific 
changes are brought about in this way, it is be- 
cause external causes have positive permanent 
effects upon the substances of which animals are 
built : they have power to change their hair, to 
change their skin, to change certain external 
appendages or ornamentations, and any other of 
those ultimate features which naturalists call 
specific characters. Now I would ask what 
there is in the substances out of which class 
characters are built that would make them less 
susceptible to such external influences than these 
specific characters. In many instances the for- 
mer are more delicate, more sensitive, far more 
fragile and transient in their material nature 
than the latter. And yet never, in all the 
chances and changes of time, have we seen any 
