334 THE INFLUENCE OF LIVING SURROUNDINGS. 



animals would inevitably result in the constancy of the de- 

 viation, and consequently the transformation of a pathological 

 phenomenon into a normal character would depend solely on the 

 uninterrupted constancy of the active causes. We ai-rived at 

 the same result when investigating those modifications of struc- 

 ture in animals which were occasioned by the first class of ex- 

 ternal conditions of life ; every variation induced by a change 

 in temperature or nutriment, in the direction or strength of a 

 current, or in the salt constituents of the water, must always 

 recur, and thus become constant or even be increased, so long 

 as the efficient causes remain unchanged. Now, in point of 

 fact, several cases have long been known to us of pathological 

 changes in animals which have become normal modifications, 

 and the causes of which can only consist in the association of 

 two species of animals. I will proceed to investigate these, and 

 a few others which are new or have met with less attention. 



A very singular genus of small corals, called Heteropsammia, 

 is found living in tropical seas (see fig. 86), of which each in- 

 dividual regularly harbours a worm, Aspidosi])hon, belonging to 

 the class of Sipunculidce. It is difficult to understand what 

 advantage each animal can derive from their association ; yet 

 some must exist, for a coral is never found without a worm. I 

 myself have fished up numerous specimens of Heteropsammia, 

 Michelini in the Philippine seas, and never found one without a 

 worm ; and in every representation and description of all the 

 species of this genus, the dwelling of this companion of the 

 coral is always found. Now, the presence of the Sipunculidje is 

 the cause of certain very conspicuous deviations from the normal 

 structure of the corals they live in peculiarities which have 

 indeed been regarded and described as specific characters of the 

 species or genus. In young specimens the base of the free- 

 growing coral is scarcely larger than the circumference of the 

 cup ; in fully-grown ones, on the contrary, it is much larger. 

 This is the first generic character which appears to be occasioned 

 by the presence of the stranger. For the intruder settles on the 

 base of the quite young coral and grows along with it ; but, as 

 it would seem, quicker than the coral, so that the worm, in 

 order not to outgrow the base in its rapid progress, has to curl 



