334 Experimental Zoology 
turely to other plants or by preventing the normal migration ? 
A few attempts that I have made in this direction show that the 
experiment is not always easily carried out, at least in the forms 
that I have used, since if transferred too soon to the alternative 
plant host the individual often dies. It would no doubt be 
easier to obtain results by preventing the normal migration. In 
the case of the fir, where migration is sometimes deferred natu- 
rally, at least for a percentage of the individuals, the cycle goes 
back to its starting point. What nature has done in this instance 
we should be able to imitate in other species by inclosing in 
gauze bags on their plants the individuals about to migrate. 
It is important to note that the different parthenogenetic indi- 
viduals of the same species that occur on the two plants may 
differ enormously in structure in some cases, as shown especially 
in the case on Hormaphis hamamelidis. The change must 
be effected not by the new plant but by the original one; for 
the parthenogenetic young may be fully developed before the 
migration takes place, but whether their subsequent changes on 
the new plant, that are sometimes very great, are brought about 
by the new plant remains to be shown experimentally. 
LITERATURE, CHAPTER XX 
Baxsiant, M. Le Phylloxera du Chéne et le Phylloxera de la Vigne, études 
d’entomologie agricole. 1884. Mém. a l’Acad. des Sci. XXVIII. 
1884. 
BLocHMANN, F. Ueber die Geschlechtsgeneration von Chermes abjetes L. 
Biol. Centralbl. VII. 1887. 
Ueber die regelmassigen Wanderungen der Blattlatisen speziell ueber den 
Generationscyklus von Chermes abietes L. Biol. Centralbl. IX. 
1889. 
Bonnet, Co. Traité d’Insectologie ou observations sur quelques espéce 
de Vers d’eau douce et sur les Pucerons. Paris. 1745. 
Burnett, W. I. Researches on the Development of viviparous Aphides. 
Proc. Am. Assn. Adv. Sci. 7th Meeting. 1853. 
CastLE, W. E. The Heredity of Sex. Bull. of the Mus. of Compar. Zool. 
at Harvard Col. XL. 1903. 
Cuotopovsky, N. Noch einiges zur Biologie der Gattung Chermes L. 
Zool. Anz. XII. 18809. 
Weiteres zur Kenntniss der Chermes-Artern. Zool. Anz. XII. 1889. 
Neuve Mittheilungen zur Lebensgeschichte der Gattung Chermes. 
Ibid. XII. , 1889. 
Cornu, M. M. Etudes sur le Phylloxera Vastatrix. Mém. a ]’Acad. de. 
Sci. Instit. de France, XXVI. 
