332 A MONOGRAPH OF THE AUSTRALIAN SPONGES, 



have caused the original water holes of the Coelenterata 

 to assume the function of taking up nourishment, whilst the 

 digestive functions have been assumed by the Epithelia of the 

 coelenteric canals. (1.) Another consequence of this change of 

 function is the wonderful variety in shape which we meet with in 

 the Sponges. Intercanal system, Pseudogastral cavities, &c, are 

 transformed in all these changes, the outer surface of the Sponge 

 always inci-eases in size, and the number of pores through which 

 nourishment is taken up is increased. The sum of all these 

 peculiarities not only favoured the development of the Mesoderm 

 but it was an efficient cause. 



This deduction only would explain why the Mesoderm in the 

 adult Sponges appears so much more developed than the other 

 layers, but it would not be sufficient to disprove the second reason 

 of Balfour's why the Sponges cannot be Coelenterata. For this it 

 is necessary to consider certain laws of heredity. 



The law of heredity in the corresponding age (Haeckel's 

 homochrone law) may be correct in general, but appeal's to be 

 modified in some cases, particularly if it is in competition with 

 settled hei'edity. 



This later kind of heredity has the inclination to let 

 peculiarities appear earlier and earlier from generation to gene- 

 ration, if these are particularly advantageous to the organism. 

 The sooner the progenxj attains j)ossession of peculiarities which 

 have shown themselves as useful for the ancestors, the better for 

 them. (If not a pure recapitulation of the Phylogeny offers still 

 better chances.) This is the main cause of the series of 

 appearances called " shortened heredity." These peculiarities 

 do by no means always appear in so striking a form, as for 

 instance, among the crustations, they form moreover a finely 

 shaded series, corresponding to the infinite variable conditions of 

 existence in the living beings. 



( 1 . ) The whole process of this retrograde development can be compared 

 to certain consequences of Parasitism . 



