276 AGRONOMY 



sowing of seeds of certain species. Following this, it has been 

 shown that many of what the botanist calls species are made 

 up of numerous simpler forms grouped around a certain type. 

 These forms are known as elementary species and agree pretty 

 closely with what the gardener calls forms or varieties. Ele- 

 mentary species may be separated out of the botanical species 

 by breeding. More than two hundred such forms have been 

 produced in Europe from the species called Draba verna. It 

 is supposed that many plants are constantly throwing off such 

 forms, but owing to the crowd of plants better adapted to 

 the locality, these seldom have a chance to mature. New 

 forms are able to persist only when they are better adapted 

 to their habitat than their parents are. It therefore appears 

 that species have arisen by either of two methods by slow 

 modifications or by sudden sports, or mutations. Having arisen, 

 however, they at once fall under the influences that result in 

 further change, and so long as their development is in harmony 

 with their position in life, so long will they exist to carry on 

 the family line. 



PRACTICAL EXERCISES 



1. In spring visit a weed patch, a neglected garden, or other waste 

 ground, and note the number of seedlings springing up. See if you can 

 discover any that are leading in the race for light and air. What do 

 you think gave them this lead V 



2. Measure off a square yard on the lawn or in a pasture and count 

 and name the different kinds of plants found growing in it. 



3. Take any abundant plant and count the seeds produced by a 

 single specimen. If each seed produced a mature plant capable of ripen- 

 ing a similar number of seeds, how many years would it take to produce 

 one plant for each square foot of soil in the world ? Why does not your 

 particular plant become as abundant as this ? Give two reasons. 



References 



Darwin, "The Origin of Species." 

 De Vries, " The Mutation Theory." 

 Vernon, " Variation in Animals and Plants." 



