200 



EXPLANATIONS. 



subject — " According to some botanists, Algae of a very 

 simple structure, placed under favorable circumstances, 

 develop and change into different plants, belonging to 

 genera much more elevated in the scale of organic being , 

 although these same algae, in the absence of su2h favor- 

 able circumstances, would be fertile, and reproduce their 

 primitive form."* I would ask if this is a point as yet 

 settled in the negative. The original of our cabbage is 

 well known to be a trailing sea-side plant, entirely differ- 

 ent from the cabbage in appearance. The cardoon and 

 artichoke are now admitted to be one, and Mr. Darwin 

 was assured by an intelligent farmer that he has seen, in a 

 deserted garden, the latter plant relapsing into the former. 



It is well known that when fresh-water mollusks are 

 exposed for a little time to an influx of the sea, those 

 which can survive the change assume considerably differ- 

 ent characters. In a fresh-water tertiary formation of the 

 island of Cos, Professor Edward Forbes and Lieutenant 

 Spratt found various fresh- water molluscan shells — palu- 

 dina, neretina, melanopsis, &c — which had passed 

 through surprising modifications in the course of three 

 successive groups of deposits, supposed to have been 

 marked by increasing influxes of sea- water. " The low- 

 ermost species of each genus were smooth, those of the 

 centre partially plicated, and those of the upper part 

 strongly and regularly ribbed, "f This was apparently a 

 retrogression to marine types. The differences in the 

 three cases were greater than those which naturalists 

 usually consider as grounds of specific distinction. 



Surely there are here ample evidences of species, or 

 what are usually regarded as such, being variable under 

 changed conditions. It will be said, these changes are 

 all mere variations of specific forms, and the facts do no- 

 thing but show that that has been called species which is 

 only variety. But where is this to have its limits ? If 

 the cabbage and sea- plant are to be now regarded as one 

 species, it seems to me that we have to go very little fur- 

 ther to come to the lines of successive forms or stirpes, 

 which my hypothesis suggests. This view becomes the 

 more striking when we remember that any variations 

 Which we now see take place within a space of time 



* Charlesworth's Magazine of Natural History, ii. f 44S. 

 t Report of the Proceedings of the British Association, 1845 — 

 Literary Gazette. 



