SoLLAS — On the Origin of Freskwater Faunas. 239 



The suppression of a free-swimming larval stage is not only met 

 with in freshwater animals, but is frequent with many marine in- 

 vertebrates. This is connected with the fact that the larval stage 

 is in a position of disadvantage as compared with the adult. On 

 the doctrine of evolution, the more complex structure of the adult 

 animal implies this. Hence there is an advantage to the organism 

 if the larval stage can be passed over, withdrawn as far as possible 

 from the chances of the environment, in a state of seclusion. From 

 this various other modifications follow; development in seclusion 

 necessarily involves a supply of easily accessible food, hence the ap- 

 pearance of yelk and other kinds of nourishment furnished by the 

 parent to the imprisoned embryo. Again, the energy of the larva 

 being economized in various directions (it is spared the drudgery 

 of working for its own existence, and is supplied with nutriment 

 in a form that makes the least demands on its digestive powers, a 

 larger balance remains available for metamorphio changes. Thus 

 arise the phenomena of accelerated and abbreviated development. 

 Further, the shortening of the larval life probably leads to the 

 lengthening of the adult life, and shifts the chances of variation 

 and selection forwards into the adult stage. Thus animals which 

 hatch out in a complete state will most probably suffer modifica- 

 tions of that state, and not of previous ones, except very indirectly. 

 Here we discover a direct tendency towards a mode of develo^D- 

 ment which explains the " arborescent " character of our most 

 natural methods of classification, i.e. the tendency of the tree of life 

 is now to produce leaves rather than new branches. In the case of 

 freshwater fauna very direct reasons have existed for the suppres- 

 sion of the free larval stage. These are to be found in the severity 

 of the climate and the dangerous direction of the river current. In 

 connexion with the suppression of the larval stage may be noticed 

 the richness in species and the poverty in genera of the freshwater 

 moUusca. 



In discussing the origin of freshwater fauna, there are three 

 hypotheses from which we have to select: 1, that marine forms 

 have migrated directly into rivers ; 2, that they have first become 

 adapted to existence in marshes, and subsequently have entered 

 rivers ; and 3, that marine areas have been converted into fresh- 

 water ones. The last holds most frequently, especially in the case 

 of attached non-locomotor forms. Hence the origin of freshwater 



