233 



when only one biotype of any form is imported to a quite diffe- 

 rent country ; where there are no other types to hand for crossing 

 purposes. Such is the case with a great number of animals and 

 plants from milder climates, which can be profitably cultivated 

 even with us, but only on condition that they are given the 

 necessary shelter, such as the tomato and poultry. 



It has long been impossible to grow citrous trees anywhere 

 but in a subtropical climate. By crossbreeding oranges and lemons 

 with one of the hardy citruses, Dr. S w i n g 1 e has of late years 

 succeeded in producing hardy trees which can be profitably 

 grown even in countries where winter brings a moderate amount 

 of frost. This is an excellent demonstration of the relative cost 

 of the two possible ways of bringing about the necessary balance 

 between genetic and non-genetic factors ; for even if the crossings 

 and extensive sowings and judging have cost a good deal of 

 money ; if once the hardy type is produced all further expendi- 

 ture ceases ; whereas the only other way to grow oranges in 

 countries where it freezes ; ist to grow them under glass, a 

 practice which would cost so much every year that it could 

 not be thought of. This relative cost of the difFerent factors is 

 an important thing in agriculture. For instance ; the average 

 length of growing season required is about equal for maize and 

 for tomatoes, Still, tomatoes can be profitably grown where maize 

 is not, simply because the value of the crop is so high that it 

 pays to grow the young tomatoplants under glass. This can be 

 done equally well with maize, so that it is possible to get it to 

 bear well in any climate, only, in relation to the value of the 

 crop, this would be too expensive. As tomatoes are somewhat of 

 a luxury and expensive, it has hitherto not been judged worth 

 while to try to breed types which could do without artificial 

 heat, or perhaps even the thing has to be stated the other way 

 round, the price of the fruits being high because öf the added 

 cost of planting out. 



Nearly in every case it will be found that it pays well to 

 find or make a biotype which is as well adapted to the cheapest 

 methods of cultivation as possible. And it will be seen that not 

 those organisms find the most extensive cultivation which command 

 the highest price per unit of area, but that the most extensively 

 cultivated plants and animals are those which as the result of 

 hybridization or spontaneously, exhibit the greatest genetic varia- 



