14 STUDIES ON TIIK GERM CELLS OF APHIDS. 



GENERAL DISCUSSION. 

 The Relation of Species. 



If we may suppose that all the species of aphids having the same 

 number of chromosomes have come from the same ancestors, the 

 mutations may be indicated by such differences in the form and size 

 relations of the chromosomes as may be seen by comparing the first 

 spermatocyte of the brown rose aphid, the Saranac willow aphid, the 

 Oenothera aphid, No. I, and the star cucumber aphid, with five chro- 

 mosomes (figs. 3, 23, 36, 105); or that of the black milkweed aphid, 

 the Oenothera aphid No. II, and the nasturtium aphid with four chro- 

 mosomes (figs. 39, 47, 66); or again, the metaphase of the maturation 

 spindle of the parthenogenetic egg of the orange milkweed aphid and 

 the pea aphid with eight chromosomes for the somatic number (figs. 

 52, 114). It is evident that mutations might as easily occur by a 

 change involving the number of chromosomes as their form and size. 



The three species having the two smallest chromosomes closely 

 associated in the metaphase of the first spermatocyte (figs. 58 and 73), 

 but differing in the number of chromosomes — seven, eight, and nine — 

 were similarly marked and probably closely related. 



The five species having five chromosomes for the reduced number 

 were obviously different in form, size, and color, while of those having 

 four chromosomes in the spermatocytes only the black milkweed 

 aphid and the nasturtium aphid were nearly enough alike to suggest 

 that they might be the same species, though one was found on milk- 

 weed at Woods Hole, Massachusetts, and the other on garden nastur- 

 tiums at Bryn Mawr, Pennsylvania. 



The two oak aphids, where the number and size relations of the 

 chromosomes are similar (figs. 69 and 76), were very unlike in appear- 

 ance when collected, one being red and yellow, the other nearly white 

 with some brown dots ; but the alcoholic specimens, minus their col- 

 oring, were strikingly alike. 



The two goldenrod aphids, with the same number of chromosomes 

 but somewhat different form and size relations (figs. 80 and 87), are 

 extremely unlike in appearance, one being a dark reddish brown with 

 green males, the other green with bright red males. 



This examination of a few out of the hundreds of species of aphids 

 has indicated to me that an intelligent classification of these insects 

 can be hoped for only when a careful study of the external characters 

 and habits of the various species is combined with an equally careful 

 study and comparison of their germ cells. My work can be regarded 

 as at present the smallest beginning of such a study, and is merely 



