ox MERLIA XORMAXI. 691 



If it be asked wliy other siliceous sponges, especially those 

 living in coral-reet areas, do not form calcareous skeletons, 

 the reply would be that it is too much to expect to be able 

 to explain why any apparently spontaneous variation takes 

 place in any living organism, because the ultimate mole- 

 cular and chemical factors elude the power of analysis. As 

 Weisraanu writes (12) — (but a propos of the trophic 

 effect of functional stimulus) — "We are here face to 

 face with the fundamental phenomenon of life, metabolism ; 

 and since we do not understand the cause of this, we ai-e not 

 in a position to say why it varies in this way or that accord- 

 ing to the stimulus." Possibly the orange-coloured proto- 

 plasmic granules of the calcocytes happened to acquire 

 under favourable conditions just that molecular constitution 

 that rendered carbonate of lime attractive, but apparently 

 under penalty of becoming petrified. Later the " penalty " 

 of the individual cells became transformed into an advantage 

 for the common good. 



The theory that Merlia constructed its calcareous skeleton 

 in comparatively recent times is rendered plausible by the 

 fact that no fossils, even with a superficial resemblance to the 

 latter, occur till Palaeozoic times. In any case the resem- 

 blance between Merlia and certain Palgeozoic fossils may be 

 purely homoeomorphous. 



If, as I believe, Merlia is a siliceous sponge^ which has taken 

 to forming a calcareous skeleton, then this sponge furnishes 

 a good example of the hereditary transmission of an acquired 

 character, and further, a striking instance of the tendency, 

 which Herbert Spencer (9) first called attention to, of one 

 part of an organism to increase in size and importance at the 

 expense of the rest. 



Weismnnn gave the name " intra-selection '^ to this teu- 



* Whether Merlia acquired its calcigenous properties in Sihman or 

 in Miocene times, it is difficult to regard it as any other than a siliceous 

 sponge related to Desmacellinse. No spongologist woxild believe 

 that Merlia got its tylostyles, sigmata, tricliodragmata and occasional 

 toxa by a process of convergent evolution. 



