NORTH AMERICAN PORIFERiE. PART II. 503 



SO generally and so uniformly in certain characteristics, by traceable physical causes, that 

 I cannot imagine the intervention of any other secondary cause ; there is no room for 

 a struggle, or for selection, since the uniform action of a given temperature, depth, 

 amount of sedunent, sheltered locality, etc., have a corresponding uniformity in results, 

 and are sufficient in themselves to account for the general modifications described. 



The Mediterranean also affords us an admirable test of the Wagnerian hypothesis of 

 an isolated locality, in which distinct races have arisen through the jDrevention of inter- 

 crossing with the parent races or sjDccies, and should give a triumphant example 

 demonstrating the truth of that theory. That distinct varieties may arise in the same 

 order on the same shore, according as they grow in shallow or deeper water, in clear or 

 muddy water, and that these differ from each other in a manner which is similar to that 

 in wliich the inhabitants of distinct regions differ from each other, is a fatal objection. 

 Again, how could it have possibly happened that all three of the Mediterranean species, 

 with their numerous varieties, arrived at the same general result, namely, greater fineness, 

 density and elasticity of the skeletal fibres, through the mere prevention of intercrossing 

 with the original coarse stock. Does it not seem at once evident, that if the coarse forms 

 of the West were the progenitors, the first settlers of the latter would have produced at 

 once in the new region all the usual variations, and that any general change in all of these 

 Avould not be, could not be a question influenced by the prevention of crossing. On the 

 same rule, the Chalinulte in any of our estuaries, ought to differ from those in every other ; 

 but they do not, unless the temperature of the water, or some other physical change, brings 

 it about. In applying AVeissmann's modifications of Wagner's hypothesis to the explanation 

 of any of the observed phenomena, I find myself equally at a loss. According to his ideas, 

 a species has a variable and a constant period in its life history ; if the isolated colonies are 

 formed during the former period a new species results through " amixie," or non-intercourse 

 with the parent species ; if the colony is formed during the constant period no change can 

 take place in this way. 



Varialiility is generally correllative with the conditions of the habitat. Upon a hard 

 bottom, or near shore, a sponge is apt to vary in a thousand ways to suit an equal 

 number of changes due to the accident of its situation, here or there, wherever the 

 larva may strike a suitable surface ; if it is upon a sandy bottom, like the Suherites 

 coinpacia Verr., it has a peculiar organization, which is remarkably dense and elastic, 

 no base of attachment, and a flattened form to enable it to rest well on the bottom 

 and be readily covered up in storms, rather than rolled about, and washed finally 

 ashore ; if it is an inhabitant of muddy bottoms then it has the anchoring threads fiist- 

 ening it in the mud, and the most invariable forms, so far as the species is concerned, 

 necessarily due to the continued influence of the uniformity of its habitat. The shallow 

 water forms, however, differ from the deep water forms ; but in the genus Tethya, for 

 example, there is the same rotund erect form, with a similar radiate arrangement of the 

 skeleton, evidently due to the necessity of anchorage downwards as well as growth up- 

 wards. I cannot, consequently, with regard to the littoral genus Spongia, imagine any 

 past or present condition under which it could become constant. Even when the conditions 

 of the habitat are completely changed, the resulting modifications evidently take place 

 immediately in the first inhabitors. Thus during an extended series of dredging in the 



