392 Causes and Course of Organic Evolution 



The Brachiopoda are so primeval a group, stand so severely 

 alone amongst other invertebrates, and are, as well as have 

 been, so thoroughly marine as far as we have any present 

 evidence, that to propose a possible fresh-water origin for 

 tliem in archa^an times may seem an utterly useless and chi- 

 merical suggestion. But, when we find a variety of species 

 belonging to the unarticulate and the articulate divisions in 

 the Cambrian rocks, this is absolute proof that these must 

 long before have descended from types destitute of shell, of 

 greatly simpler structure, and also of much smaller size. Such 

 seem to be represented embryologically by Beecher*s typembryo 

 and phylembiyo, that suggest highly modified trochosphere 

 derivation as indicated by the studies of Kovalevsky and 

 Morse. But even then we have no evidence that the group 

 had not become marine. So the Brachiopoda, like the Echino- 

 dermata, many Polyzoa, and three groups of the Mollusca 

 seem all to have acquired their group characters in littoral 

 or marine surroundings, during some period in the late archsean 

 epoch. But such in no wise militates against their earlier 

 trochosphere stage having originated in and been derived 

 from species originating in land-locked areas of the mid- or 

 late-arch?ean age. The question is more fully discussed in 

 later pages (pp. 518, 520, 528). 



The group is now represented by 15 genera, a poverty in 

 numbers that forms a striking contrast to their wealth of 

 genera, and particularly of species, during the silurian and 

 devonian periods of the earth's history. 



The large cosmospohtan group of the Mollusca, like that 

 of the Echinodemiata and of the Brachiopoda, seems emi- 

 nently to favor the view of a marine origin for life. So Cooke 

 in the opening sentence of his volume on "Mollusca" {133: 1) 

 says: "It is the generally accepted opinion among men of 

 science that all life originated in the sea." Throughout his 

 work therefore this view forms the leading thesis for an expla- 

 nation of the distribution of living molluscan forms. If an 

 appeal be made to the pala^ontological record, one is struck 

 by the fact th^t all of the main molluscan groups can be traced 



