192 Journal of Agricultural Research voi. xxii, no. 4 



the nematode Heterodera schachti Schmidt). After pointing out the 

 interdependence and relation of the two, M. Capus says: 



One might ask himself why this species, everywhere known for its injury to beets, 

 does not establish itself on this plant in the Gironde and appears so abundantly 

 on peas. 



Following M. Capus's explanations of this phenomenon. Dr. Marchal 

 observed : 



Among the very interesting facts pointed out by M. Capus in his note I wish to 

 call attention to the following: That injury to beets by Heterodera in the Gironde is 

 not constant, is rare. We should recall in this connection the observations, already 

 old but interesting, of the Dutch naturalist Ritzema-Bos. He has shown that when 

 nematodes multiply in course of years without interruption on the same host, biologic 

 races are formed adapted to this host which later pass to other vegetation with greatest 

 difficulty, even when these are of those preferred by the species. It must be, there- 

 fore, that, by virtue of the conditions of pea culture in the Gironde, a race of Heterodera 

 schachti was formed especially adapted to peas and to the attack of which beets are 

 resistant up to a certain point. There is no doubt that it will adapt itself to beets 

 cultivated for a number of years in succession in the same soil infested with H. schachti. 



The experiments conducted at the Gipsy Moth Laboratory^ show that 

 of the many plants tested a decided variation was found in regard to 

 the susceptibility to attack by this insect. The plants were divided into 

 four groups: I, favored species; II, favored food species after early 

 stages; III, species on which a small proportion may develop; IV, species 

 that are unfavored food. These results show that, although this insect 

 has a wide variety of hosts on which it is capable of feeding, certain ones 

 are selected in preference to others in the natural forests. As far as 

 known, no observations have been reported showing whether or not 

 several years' feeding on any particular host produces a strain which 

 selects that in preference to others. 



In a recent paper Dr. C. T. Brues^ writes as follows (on p. 328-329): 



It has been claimed that the food habits may be modified experimentally, in that 

 caterpillars reared on a strange plant (where they could be induced to select it) give rise 

 to moths whose progeny more readily accept the new plant. It is very difficult to 

 accept such evidence, at least as having any general application, without very clear 

 and incontrovertible proof. If such transformations can occur so easily and become 

 hereditary so quickly they should have entirely destroyed the coherent habits now 

 existent, during the enormous period which has elapsed, for example, since the 

 violet-feeding Argynnids were differentiated, since the holarctic and nearctic Vanes- 

 sids have been separated, or while the world-wide Aristolochia-feeding Papilios were 

 attaining their present distribution. That such a change has actually occurred in 

 the caseof other groups seems equally evident, althoixgh, as has been shown, we can 

 more easily believe that they may have arisen through mutations in maternal instinct 

 not incompatible with larval tastes and then only in extremely rare cases and con- 

 fined to certain groups. 



METHODS OF CONDUCTING EXPERIMENTS 



In connection with the experiments by the writer several types of 

 cages, the particular type determined by the amount of material handled 



> MOSHER, F. H. FOOD PLANTS OF THE GIPSY MOTH IN AMERICA. U. S. Dept. Agr. Bui. 250, 39 p., 6 pi. 



2 Brues, Charles T. the selection of food-plants by insects, with speoal reference to lepi- 

 DOPTEROUS LARVAE. /» Amer. Nat., V. 54, no. 633, p. 3i2-33"2. 1920. 



