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of the expression "typical,'' as used by Mr. Collinge, but in reply to my question, he writes that in every case it was "the pure black form, not nigrescens or plumbea."

(2) castanea, a brown variety.

(3) rufa, a red variety. The reddest forms occur on the continent of Europe. The above three have the body unicolorous, though the foot fringe may vary.

(4) albolateralis, with the back black and the sides white, the two colors sharply separated. A very handsome variety, of restricted range, especially common in Wales. Mr. Collinge writes me that the specimens he used were found in the vicinity of Birmingham.

(5) scharffi, like albolateralis, but the sides yellow instead of white.

In the first experiment, castanea was paired with ater, and the former laid 39 eggs, of which 24 were hatched and raised to maturity. These proved to be: 12 ater (with slight variations in foot fringe), 10 rufa, 2 castanea. From this lot, rufa was paired with castanea, and gave 14 in the next generation, of which four were ater, eight rufa and two castanea.

From the pairing of two of the eight rufa, fifteen slugs were raised, eight being rufa, two ater, and five subvarieties of castanea. (Mr. Collinge does not explicitly state eight rufa, but in a letter he confirms this interpretation of his account.) From the pairing of two rufa of the last generation sixteen adults were raised, twelve being ater, two subvarieties of rufa, and two subvarieties of castanea. Thus the experiment was carried to F,, with results which are thoroughly Mendelian so far as the segregation of characters goes, but difficult to explain in regard to the appearance and proportions of the different kinds. It will be noted that rufa was twice chosen for breeding, to the exclusion of the other varieties, and the second time gave many more black slugs than the first. None of the bicolored forms appeared.

In a second experiment two albolateralis were paired, giving a progeny of 22 slugs, 20 being ater and 2 scharffi. This is so extraordinary that I asked Mr. Collinge particularly about it, and he confirms the result as stated. One would expect albolateralis to be homozygous, but the experiment shows that it is either heterozygous (in which case the proportions are hard to explain) or the results are incapable of explanation by any ordinary hypothesis. I can not help suspecting that the parent slug had really paired earlier with a specimen of ater, the progeny in consequence not actually having the origin stated. In that case, supposing ater to be dominant, the results would not be so anomalous. It is not so easy to explain the results of the first experiment by the hypothesis of previous pairing, as in that, except for the original pair, the slugs were under observation from their birth.

Seeking a possible explanation of the albolateralis case, I consulted Dr. C. B. Davenport, who practically concurs with my view, writing: “The result of Collinge's mating is inexplicable to me except upon one or the other of two hypothesis; either that the parents were heterozygous or else, as you suggest, the supposed parents were not the actual ones, and one had previously paired with a black slug. Of course if albolateralis is a heterozygote, then striping is dominant as is usually the case and uniformity recessive. Uniform black would then be active in one quarter of the offspring, but the great proportion of pure black speaks against this hypothesis” (litt., March 22, 1909).

T. D. A. COCKERELL.

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