94, PLANT LIFE ON THE FARM. 
The limits of variation no one can tell; sometimes they 
seem very narrow ; at other times we know them to be 
very wide. Within short periods of time the amount of 
variations may be inappreciable. Within geological 
periods the variation in the course of development is 
sometimes so enormous that, were there not evidence of 
the fact, it would be difficult to connect the plants and 
animals that have gone before with those which now 
exist. Living plants, then, are influenced in the course 
of their development by two somewhat antagonistic prin- 
ciples—the hereditary principle which, on the whole, 
tends to keep plants as they are, and the tendency to 
vary, which is the source of that variation in character 
which enables plants and animals gradually to become 
adapted to altered circumstances. This is beneficial to 
the cultivator, by affording him an opportunity of select- 
ing the varieties best suited for his purpose. It is by 
exercising selection of this kind that Mr. Hallett suc- 
ceeded in raising his ‘‘ Pedigree Wheat.” He simply 
selected for sowing the best grains from the largest and 
best ears, and repeated the process year after year, just 
as the gardener has done for countless generations in the 
case of fruits and seeds, as the cattle-breeder does with 
Shorthorns or other pedigree animals. These processes 
the farmer whight with great advantage practice to a 
much greater extent than he usually does, and thus se- 
cure hardy productive varieties best suited to his particu- 
lar conditions and. best likely to fulfil his requirements. 
Formation of the Embryo.—The two processes of 
growth and development may also be illustrated by recall- 
ing what takes place in the germination of aseed. A 
ripe seed contains within its coat or husk an embryo 
plant. Very often that embryo plant is invested, as in 
the case of the grain of wheat, with a whitish, floury 
substance, known as the ‘‘perisperm.” All grass seeds 
