1 8 THE BIOLOGY OF THE SEA-SHORE 



the open sea or in the neighbourhood of the coast. Simroth 

 considers the choice must inevitably fall upon the latter. 



With regard to the pelagic origin of life at a time when 

 the earth was entirely surrounded by water, as postulated by 

 Marshall, and later, as we have seen, by Church, Simroth 

 argues that since the earth first began to solidify at a tempera- 

 ture far greater than that at which it was possible for life 

 to exist, a disproportionately longer time must have elapsed 

 between the first formation of a terrestrial crust and the 

 appearance of life, than from this latter period till now. 

 There seems no reason, however, why the earth's crust 

 should not have become folded and portions of it have pro- 

 jected above the water surface, long before it could have 

 cooled to a temperature favourable to life ; and conse- 

 quently the theoretic possibility of a coastal or even a land 

 origin of life cannot be excluded. 



SoUas (1884) assigns a marine origin to life and brings 

 evidence to show the probability of fresh-water faunas having 

 arisen by modification of marine forms following the con- 

 version of shallow continental seas into fresh- water lakes. 

 The fact that practically all marine forms have free-swimming 

 larvse makes their direct transference to rivers extremely 

 difficult, since the larvse are at the mercy of currents. Sollas 

 enumerates three considerable methods by which fresh- 

 water animals may be derived from marine forms as 

 follows : (i) direct migration into rivers from the sea ; 

 (2) conversion of the original habitat into a fresh-water 

 basin or lake ; (3) acquisition of a terrestrial or marshy 

 habitat and subsequent exchange of this for a fluviatile or 

 lacustrine one. It is interesting to compare this with the 

 observations of Pearse (191 5) on the migrational highways 

 utilised by Crustacea. In dealing with the Crustacea of 

 the southern shores of the Caribbean Seas, and the varying 

 habitats : sea, mangrove swamps, streams, forest, and desert, 

 this writer shows that this group, like the Amphibia, is in 

 process of transition from aquatic to terrestrial life. Every 

 phase of the transition is shown : marine, fresh-water, ter- 

 restrial, arboreal. The variety of habitat is facilitated by 



