20 The Animal Ecology of the Cold Spring Sand Spj«t 



This law is illustrated again by an inhabitant of the upper beach, Oniscus. 

 Oniscus seems, on the whole, rather poorly adjusted to a terrestrial life. Its gills lack 

 the well-developed tracheal chambers of the wood louse, Porcellio, and the pill bug, 

 Armidillidium, its close relatives (Stoller, 1899). Correspondingly we find it only in 

 moist situations, under logs, in cellars, in greenhouses, etc. On the other hand, Por- 

 cellio ratzburghii, Brandt, is a species in which all five pairs of outer gills possess tra- 

 cheal chambers. As Stoller (1899) remarks: " This species lives in situations where 

 the air is charged with moisture only in a moderate degree in excess of that of ordi- 

 nary atmospheric air. Their habitat is under the bark of dead trees, and they may 

 often be found a meter or more above the ground." Now, experiments that I have 

 made show that a water-inhabiting isopod (Asellus), if taken out of the water, will go 

 back into it if free to do so; and Porcellio, if put in water, will leave it for dry 

 land, if free to do so. Similarly we may conclude that Oniscus chooses a situation 

 that affords the requisite moisture. Shall we conclude that the reason why Oniscus 

 has no tracheal chambers is the result of its living in a moist situation, or is it the 

 cause? We find it where it is because it is hydrotactic. Now, is it hydrotactic 

 because it has no lungs, or is it without lungs because it is hydrotactic? Certainly it 

 would be rash to assert the latter, even though we cannot prove the former. 



So likewise we find Nassa, which has a siphon to enable it to draw pure water 

 from above the mud, living in the mud; whereas Littorina, which has no such siphon, 

 clings to the stems of the marsh grass above the mud. Can we say that Littorina has 

 no siphon because it clings to the marsh grass, or does it cling to the marsh grass 

 because it has no siphon ? I maintain that the latter is no less true than the former. 



Let us consider still one other case. We have seen that the mussels cling to the 

 banks of the channels in such numbers as to make a protecting wall. Of the advan- 

 tageousness of that situation for the mussel as a lamellibranch there can be no doubt; 

 abundant food and oxygen are brought to its doors every day. The wonder is that 

 no other lamellibranchs than the mussel occupy this favorable situation. Why is this ? 

 It is because the mussels are the only species living about the sand spit that have a 

 byssus in the adult stage. Lacking a byssus, the other species cannot attach them- 

 selves to the banks. Now, does Mytilus have a byssus because it tends to attach itself 

 to banks, or, being provided with a byssus, was it led to take advantage of the favor- 

 able position offered ? Did the situation or the organ precede ? In this case we may 

 see, I think, the necessity of the organ being well developed before the special habit 

 (of attachment) could be exercised. However, it is quite likely that the byssus has 

 been improved in the race by selection, and with every step of improvement the race 

 has been able to take up a more and more advantageous habitat. 



This brings us, indeed, to the most reasonable hypothesis of adaptation, namely, 

 the combination of the improvement of the organ to meet the requirements of the 

 environment, through selection, and of improvement of situation to meet the abilities 

 of the organism. There have gone on, hand in hand, a selection of more appropriate 



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