Gatty ^farine Laboratory, St. Audrciot. 159 



aeration, plays an important part in the economy of tlie 

 annelid. 



In such a type as Fi/of/rana it is not tiie hard-and-fast 

 rule that "like begets like," but the inherent tendency to 

 vary in every particular in the parts indicated is the main 

 factor. It is questionable if, with every care, the production 

 of those with opercula or those with the greatly enlarged 

 tips to the l)ranchial filaments could always be relied on by 

 breeding from suitable parents of each type, unless the 

 exact surroundings are obtainable, since the tendency to 

 variability is so intense. It is said that varieties replace the 

 original species because they are " more perfectly developed 

 and more liighly organised, and in all respects better 

 adapted to secure its safety, and to prtdong its individual 

 existence and that of the race. Such a variety could not 

 return to the original form ; for that form is an inferior 

 one, and could never compete with it for existence. Granted, 

 therefore, a 'tendency ' to produce the original type of the 

 species, still the variety must ever remain preponderant in 

 numbers, and under adverse physical conditions again alone 

 survive" *. 



"Whilst many examples of the foregoing statement may be 

 found in the higher vertebrates, the ease of Filograna, for 

 instance, does not seem to fall into line, for here are variations 

 so numerous in structure and development and so inter- 

 mingled with each other that it is difficult to say which is 

 the original form and which the variation, since all forms 

 may be found under like conditions. In this connection it 

 may be asked what variety of Filograna has a tendency to 

 maintain its existence longer than the original species or 

 longer than any other variety ? Can it be said tliat those 

 with opercula are better fitted to survive than those devoid 

 of them, or that those with the distal ends of the branchial 

 filaments enlarged into sausage-like masses supplant those 

 without them ? Are those with eyes and nine pairs of anterior 

 setigerous processes enabled to continue the species more 

 eftectively than those which have no eyes and only five or six 

 setigerous processes? There is no proof that any of these 

 is in a better position than another — yet Nature does nothing 

 in vain; the facility wiih which variations occur and the 

 vast distribution of the species would lead to the belief that 

 a clue may yet be found to unravel the mystery. The 

 species certainly fluctuates to and fro in regard to the organs 

 mentioned, but does not progress along any of the lines 



• Tl 

 Mr. Darw 



lifl find other quotations aro talion from the iniportiint pnpers of 

 irwin and Mr. Wallace in the Fruc. Linn. yoc. vol. iii. no. 9, 18G8. 



