CONGRUITY OF THE EVIDENCE 305 



with their forms and structures than any differences in the ' con- 

 ditions ' under which they have been born and have Hved — a view 

 which is supported by the fact of the similarity of the forms of 

 Diatoms and of Foraminifera that existed upon the surface of the 

 earth or in its oceans in remote ages with those hving at the 

 present day. 



But are we to assume that these and other low organisms to 

 which reference has been made have had an unbroken lineal descent 

 through millions and millions of years since they iirst came into 

 existence ? This is the commonly received notion, in accordance 

 with the view that Archebiosis no longer occurs, and that Hetero- 

 genesis is equally to be denied.' In view of the extreme variability 

 of all these organisms, and in face of the fundamental principles 

 of Evolution the supposition will, perhaps, to most persons prove 

 absolutely incredible. If lineal descent without alteration had been 

 the rule in past ages and up to the present day, we might at least 

 expect now to find such evidence of this continuity as would be 

 represented by fixity of habitat. Fixity of habitat is, however, 

 notoriously non-existent among these lower forms of life. Speaking 

 of Rotifers, Pritchard says " : " One remarkable circumstance must 

 be borne in mind by the animalcule hunter. If he happens to 

 remember a pond where some rare species abounded last year, let 

 him not again turn thither in search of it, as the chances will not 

 be in his favour. These creatures rarely exist in the same water 

 during two successive years. The reasons for this are not easily 

 ascertainable. The remark is equally applicable to Volvox and 

 the Desmidiae. The search will be most productive if prosecuted 

 on new ground." Statements of this kind have also been made 

 by others and are fully borne out by my own experience. 



Ralfs, for instance, in reference to some Desmids, says 3 : " The 

 Staurocarpus cceruleus is not uncommon near Penzance, is generally 

 in large quantity where it occurs, and from its pecuHar colour 

 cannot escape detection ; on these accounts I made it a principal 

 subject of my observation. Although I have yearly gathered it in 

 several pools, and the sporangia are always abundantly produced, 

 I have particularly noticed during five or six years' observation 



• Huxley, for instance, said, " In view of the facts of geology, it follows that all 

 living animals and plants are the lineal descendants of those which lived long 

 before the Silurian epoch." 



' hoc. cit., p. 655. 



3 " The British Desmidae," 1848, p. 14. 



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