DISTRIBUTION OF SHORE ANIMALS. 329 



animals, if we could not give a reason why pelagic larvae 

 showing adaptive simplicity should occur in the life-history 

 of shore animals. But a twofold reason is fairly obvious, 

 and has already been suggested by implication. Shore 

 animals usually have armour, are often sedentary, are rarely 

 strong or swift swimmers: the minute active larvae ensure 

 distribution ; in their own sphere they fulfil the same 

 function as the winged seeds of our great forest trees, and 

 their occurrence in the life-history is justified by this fact. 

 Again, Prof. W. K. Brooks (see his Foundations of Zoology 

 for details) suggests that this occurrence is also justified by 

 the fact that life on the whole is less precarious in the open 

 sea than near the shore. We have repeatedly emphasised 

 the fact that in the shore waters there are multitudes of 

 sedentary animals who live upon minute creatures found in 

 the water, and who are constantly creating miniature whirl- 

 pools in the water as they lash it through their bodies. 

 Against such maelstroms the young forms would have no 

 chance, so that it is safer for them to acquire more and more 

 purely pelagic characters, and get out into the open where 

 there are not so many hungry mouths ever ready for food. 



We thus see that the arguments for the theory of the 

 pelagic origin of littoral animals seem to be nearly balanced 

 by the arguments against. Does the converse theory that 

 pelagic animals originated from littoral fare any better ? The 

 theory may be put in this way. Littoral animals send off 

 pelagic larvse out into the open, and the specialisation of 

 these larvae takes place along different lines from that of the 

 adults ; the larvae acquire elaborate mechanisms to keep 

 themselves afloat, forms of armour which may protect them 

 without adding greatly to the body-weight, such pale and 

 delicate colours as may render them inconspicuous in their 

 uniform background, and so on. Is it possible that long ago 

 some of these larvae forgot to grow up, if we may put the 

 matter so, and gave rise to the original pelagic animals 1 Is 

 the resemblance between pelagic animals and the pelagic 

 larvae of littoral animals due to the fact that the latter or 

 similar forms were long ago the ancestors of the first, in- 

 stead of to the converse relation 1 We shall not follow the 

 question in further detail perhaps to some it may seem to 

 be identical with the momentous question whether the egg 



