Eeactions and Products in Interspecific Crosses 93 



Frequent sterilization of the pots, soil, and cages by live steam under a 

 pressure of 10 to 20 pounds for periods of 4 to 10 hours for each sterilization 

 has materially contributed to freedom from disease, parasites, and kindred 

 troubles in both the animals and in their food plants. For this purpose steel 

 drums have been used. These are about 3 feet in diameter and 4 to 6 feet long, 

 and are connected to the steam mains that heat the quarters used for the experi- 

 ments. Live steam enters at a pressure of 00 pounds and with a temperature 

 that ranges from 163° to 170° C, and then dropping to a temperature of 108° 

 to 113° C. for the temperature of the sterilizer. 



With properly sterilized cages, good food, and healthy stock, the chances of 

 numerous and vigorous progeny are as good as in nature, and I have often sus- 

 pected that in my cultures the progeny really surpass in number and in vigor 

 their less favored brethren in nature. Once a culture is set up, the maintenance 

 thereof is not difficult nor time-consuming. Only a moment is required daily 

 in which to thoroughly spray the cage and the food plant with the hose and to 

 add to the soil a fairly regular amount of water in order to maintain optimum 

 conditions of growth. This in general is the method of handling pedigree 

 cultures that I have used, and it has uniformly given satisfactory results. 



I have been especially interested in the relation of the Mendelian reaction to 

 the problems of species and groups in nature, and the question whether in the 

 crossing of species a special order of reactions is shown. All of the older workers 

 seem to be of this opinion. De Vries clearly thinks of the Mendelian reaction 

 as present only in those forms standing in the relation of species and variety, 

 while elementary species or specific groups present a different type of reactions. 

 The same conclusion is expressed by many observers as the result of diverse 

 experiences. 



The materials that I have used all come direct from nature, have never been 

 under domestication, nor in any way subjected to the operations of man. They 

 are natural species, and some of them have been known for more than a half 

 century. With this material I have made sundry studies to determine whether 

 in the crossing of species the reactions and laws of action were different from 

 those found in the lesser groups in intraspecific crosses. I could discover no 

 a priori reason why this should be so ; but the common expression of opinion that 

 it was so made the test necessary for the investigations that were in progress. 



Species differ in diverse, concrete qualities ; some that are not easily expressed 

 in terms of " character," as for example, the " sense of difference," one soon 

 learns to recognize, even though one is not able to state it in accurate terms. 

 There is the total type aspect, with which are associated certain more or less 

 easily described characteristics. I have been so fortimate as to find in the 

 material that I have used species that could live with success under the com- 

 plexes provided in my laboratory, and that showed in these interspecific crosses 

 an array of characters such that it was possible to get a picture of the opera- 

 tions in the crossing of species. The general outcome of the entire series is 

 that in this set of organisms there are no differences in the crosses of species 

 that differentiate the reactions between species from intraspecific crosses. In 

 all respects the principles of factorial composition and reaction are fully con- 

 firmed, and many instances hopelessly confusing at first, and contrary to the 

 findings in intraspecific crosses, have been solved, in no instance with any dis- 

 cord in principle. 



