Such plants as the little shepherd's purse and the brake-fern 
occur in the temperate zones of both hemispheres, and are easily 
identified. But some species undergo radical changes in appear- 
ance as environmental conditions change, just as certain varieties 
of the pink-flowered hydrangea becomes blue-flowered when 
grown in soil containing alum, and a certain variety of primrose 
with red flowers has only white flowers when the temperature in 
which it grows is raised fifteen degrees. 
Variation is so rife among the characters of some plants that 
even the greatest specialists on classification find great difficulties 
to so classify them that one of them will understand what the 
other is talking about. For example, the genus Rubus , to which 
our blackberries and raspberries belong, contains, so some author- 
ities say, 1,500 species, while others classify the same material 
into 200 groups. 
When the attempt is made to call all the individuals spring- 
ing from seed of the same plant a species, variation again causes 
trouble. Hybridization is extremely common among all cross- 
fertilized plants, and the characters of two closely related species 
which cross are shuffled and form new combinations with each 
other, so that often a motley array of progeny is produced when 
seed is sown. Seed from botanic gardens often excellently 
illustrate this, but the same result will happen in nature where 
the species occupy the same general territory. Variation in 
species, as previously noted, arises also in other ways. Many 
new types arising in nature may be lost unless extremely favor- 
ably situated and this contributes toward keeping species stable. 
The same species of wild plants that apparently breed so true in 
nature, when brought into cultivation under the watchful eye of 
the gardener, in most cases become extremely variable— perhaps 
because the gardener is on the watch and gives the new variation 
a chance. At any rate, wild species under cultivation generally 
break up into a large number of forms so strikingly distinct from 
each other, that they would be given specific rank had they been 
found wild. Take the sweet pea, the purplish-red flowered wild 
ancestral species of which is native to Sicily. Occasionally white- 
flowered sports are found wild. The varieties of sweet peas are 
legion now, and many would be accorded specific rank unhesitat- 
ingly were they found wild. The same is true of the petunia, 
which ancestrally traces its origin back to two wild South 
American species. The variation that has come about in the 
chrysanthemum since its first domestication by man is perhaps 
the most marvelous of all. The supposed wild species are two, 
one with small white and one with small yellow flowers. The two 
species are very similar in other respects. While some plants 
have varied greatly since man has had them under observation; 
others, perhaps cultivated equally as long and observed just as 
carefully, have given rise to practically no variations. This is true 
