I. AS On the Oriyin / /', ,-*// n;,,',r l\,,n 117 



tin; tin 'icr ymin^ ;i sanguineous fluid which is probably similarly derived. 



Kven when tin- diet is so digestible as fish, the voting bird is not put to feed upon 

 it al\\;i\> in the unprepared state, but ifl ;illo\ved to take it from the erop of the 

 parent where it has probably undergone a partial digestion. 



The feeding of youug pigeons with milk has been hitherto explained by the 

 comparative helplessness of the young bird; but in that case what necessity is tli 



lie -penal secretion of milk? One would have thought a grain diet could have 

 provided for it by the parent at a smaller expense to the parental resources. 



We baft MOW to end this digression by pointing to one further consequence 

 which follo\\- upon the comparatively late assumption of an independent e> 

 in the case of the higher animals. It is that in seclusion, where the play of 

 ineidei as little varied as possible, variations from the usual course 



'in-lit are unlikely to occur, and consequently the variations which so con- 

 stantly appear in a species are, as a rule, to be traced to the action of iiu id 

 forces on the adult animal. A form that passes through a free larval development 



iable to modification at any point of its development: one that does not is 

 fhi i 'ble of modification after birth. Hence, as no crustacean app- 



upon the world at an earlier stage than a nauplius, so no crustacean can dive 

 I'ruin tl, phylum at a point lower down than that of the nauplius; while the 

 lob- n fully formed, cannot develop into fresh varietal modifications 



exci ulded on the lobster type. This helps us to understand the 



like form of zoological classifications. 



Returning now to the viviparous character of the development of Paludina, we 



1 that it is not an isolated case amongst the Mollusca, for it occurs also among 



Murcxe- ; and further, that it is an illustrative case of a general tendency 

 toward- ! velopuient in seclusion, the advantages of which are in most cases great, 

 but particularly so in relation to a freshwater mode of existence. 



The views here advanced as to the origin of freshwater faunas may be briefly 

 summarised as follows : 



1. The conversion of comparatively shallow continental seas into freshwater 

 lakes has taken place on a large scale several times in the history of the earth; 

 and has been accompanied by the transformation of some of the marine into fresh- 

 water spec! 



2. At their inception these species may have been phanerogenous ; if so, 

 they have since nearly all bee<nne cryptogenous. This change has been accoin 

 plished by the .-elective action of climate and river currents on varietal modifica- 

 tions induced by change of climate (Hydra, Spongilla), or of food (? Palud; 



itic glochidium), or otheru 



3. While apparently the only probable explanation of the origin of sedentary 

 freshwater forms, the hypothesis will >. , utlly well to account for 

 which. motive: though in their < jilauution is possible, that 

 of direct migration from the I 



