233 
when only one bio-type of any form is imported to a quite cliffe- 
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 i)ays well to 
find or make a biotype which is as well ada[)ted to the cheapest 
methods of cultivation as possible. And it will bc seen that not 
those organisms find the most extensive cultivation which command 
the highest price per unit of area, but that the most extensivoly 
cultivated plants and animals are those which as the result ot" 
hybridization or spoiitaneously, exhibit the greatost geiietic varia- 
