THEIR STRUCTURE AND MIGRATIONS. 
19 
species of Potamogeton. These plant-movements, which have for 
their object the protection of the seed while it is ripening, are very 
curious and interesting. 
The ways adopted by nature to secure dispersal of seeds over the 
face of the earth will well repay study. The great importance 
attaching to this subject of seed-transportation will be evident 
when two things are considered—first, the great advantage to be 
derived from a rotation of crops; and secondly, and even more 
important still, the fact that there is going on everywhere a 
great struggle for existence which necessitates the emigration of 
seeds, if I may use the term, to places better fitted for their germi¬ 
nation. There is a continual fight for life taking place in the 
plant-world just as there is in the animal kingdom. The indi¬ 
vidual that is best fitted to its environment survives, the weakest 
goes to the wall. There is not room for all, some must perish. 
All through the ages this struggle has been raging, each year 
with greater fierceness as the competition has increased, and the 
result has been that the strongest has survived. It is a struggle 
not merely between different species but between individuals of 
the same species; each seed a tree produces enters into competition 
with every other seed of the same tree. When we come to con¬ 
sider the high rate of increase that prevails among all organic 
beings we shall understand how necessary as well as fierce this 
struggle is. Linnaeus has calculated that if an annual plant pro¬ 
duces so exceedingly few as two seeds a year, and their seedlings 
next year produce two, and so on, in twenty years there would be 
a million of descendants ; and if instead of two the seeds yielded 
were a hundred, or it may be even a thousand, and the plant kept 
on increasing at the same rate, in the course of a few years there 
would be no room for any other species to exist. 
Mr. George, of Park Street, informed me a few summers since 
that the produce of one single bean grown in his garden was 182 
pods, containing 389 beans. This of course for a bean is a re¬ 
markable case, but knock off the odd 89 beans as unripe, or un¬ 
productive from any cause, and it will be found that if every one 
of the 300 beans left went on producing seed in the same ratio, at 
the end of six years the offspring of that one single bean would be 
two billions 430 thousand millions of individuals. A bean is a 
large-seeded plant, and does not yield seed in anything like the 
number of some smaller plants, so that if numerous enemies were 
not at work to check this increase the result would be something 
alarming. With every species increasing at so. great a ratio they 
can only be kept down by this continued struggle which is going 
on. Species, perhaps at quite different ends of the chain of 
organic existence, depend upon one another for help, or are kept in 
check the one by the other as the case may be. Mitigate the agents 
of destruction, or increase the friendly form ever so little, and a 
species will almost instantaneously increase by leaps and bounds. 
Any number of illustrations of this complex dependence of 
different species could be given, but take only one. Large tracts 
