On the Variations of certain Crustacea. 335 



and the nearly related D. Stromii, of Heligoland, likewise 

 shows a similar tendency to variation. Circmnstances of this 

 kind prove that closely-allied species are subject to changes of 

 a similar nature. Bactylopus Stromii seems especially liable 

 to great divergence. The seventh antennal joint is of very 

 uncertain length, and is even sometimes divided into two 

 joints. This Dr. Clans has observed only in one instance, the 

 antennas having nine, instead of eight joints, and in their 

 relative size simulating those of the allied genus Thalestris. 

 Of thirteen known species of JDactylopus, far the most have 

 eight-jointed antennas, two species have ten joints, one has 

 seven and one five. JVme-jointed antennas are not met with 

 normally in any species of Dactylopus. Thus it appears that 

 through individual alteration characters may be acquired which 

 one is accustomed to regard as being even of generic import- 

 ance. In the same way, the Mediterranean species of Har- 

 pacticus have m'w-jointed antennas, the northern species being 

 eight-jointed. Greater and smaller individual differences may 

 occur in all parts of the body, and these often occur in such 

 wise that though there may be no intermediate forms, it is 

 quite impossible to regard the animals as belonging to distinct 

 species. We must in such cases allow the influence of " time's 

 effacing fingers/'' Highly interesting and important in this 

 view are the variations in the length of the third and fourth 

 antennal joints of Harpacticus nicaiensis. The stronger, and, 

 on the average, the larger form, has a heavy, strong body, 

 ill-bred apparently, inactive and wanting in mobility, the 

 antennas clumsy, with their third and fourth joints short and 

 thick, the second joint very long ; the second foot-jaw ends in 

 a strong, massive, clasping hand ; the first pair of feet are 

 armed with doubly-curved claws ; the feet, especially the last 

 pair, are strong and clumsy, all the setae showing a tendency 

 to become plumose. The smaller and slenderer breed has 

 longer antennas, the third and fourth joints of which are much 

 elongated ; the prehensile apparatus of the foot-jaws and first 

 pair of feet more slender, and there is also a much slimmer, 

 slenderer form of the limbs. In general structure and con- 

 formation of body, in the peculiar arrangement of setas, the 

 serration of the abdominal segments, in short, in those points 

 where distinct species mostly diverge, there is here a striking 

 agreement. " After diligent inquiry," says Dr. Claus, " these 

 differences remained unexplained, and I was inclined to con- 

 sider them as mere individual variations. But further inves- 

 tigation of all portions of the body convinced me that two 

 distinct forms, with qualities diversely useful, had originated 

 two separate races, one slender, swift, and agile ; the other, 

 clumsy in figure, but robust and powerfully armed. The two 



