52 



The Unity of the Organism 



consult such systematic monographs as those of Schulze 

 and Sollas in the reports of the Challenger Expedition. A 

 picture of such a group of spicules as that shown in figure 

 48 reminds one of pictures of ice crystals he has seen; and 

 the question may well be raised, Are not these spicules in 

 reality crystallization forms, and hence as devoid of hered- 

 itary significance as are snow flakes? The fact that the 

 form they have depends on the particular group of sponges 

 to which they belong, i.e., that they follow the rules of 

 biological taxonomy, is very strong evidence that they are 



FIGURE 49. DEVELOPMENT OF A SPICULE (aFTER LANKESTER). 



genuine organic productions, and not mere crystallizations. 

 And the further fact that they follow this rule even though 

 many of them appear to have no functional significance in- 

 dicates that their particular forms are due to heredity and 

 not to modeling by extraneous influences in each individual 

 sponge. But we are not left to such general evidence for 

 support of the supposition that they are true organic 

 products and subject to heredity. Their development has 

 been studied by several zoologists and the results leave no 

 doubt about their nature so far as this point is concerned. 



A single instance will be enough for our purpose, but it should 

 be remarked that many others could be given. This is taken from 

 the excellent summary of what is known about sponges written 

 by Minchin for Lankester's Treatise on Zoology. "To form a 

 triradiate spicule three cells migrate into the parenchyma from 



