OEIGIN OF LOCAL FLOEAS. 
31 
I and liave to be raised every year from seed. But we bave 
historical evidence that man has cultivated them for upwards 
of two thousand years^ and also that the species has not 
changed during the whole of that period. The present crops 
have therefore been preceded by 2^000 generations of the same 
I species. Now suppose an equal number of generations of 
I oaks and pines to have preceded those at present growing in 
England — and what is there unreasonable in such a suppo- 
sition ? — if we give to each generation only an average duration 
j of 100 years^ then we get a period of 300^000 years during 
! which oaks and pines have been in existence. But leaving all 
f calculations as to the age of trees out of consideration for the 
I present^ plants with herbaceous sterns^ which are annually 
I perishable_, whether dicotyledons or monocotyledons^ have 
r precisely the same growth_, as far as it goes, as those which are 
i. ligneous and persistent. Any one can speedily convince him- 
’ self of this. In the case of the stems of dicotyledonous 
I herbaceous annuals_, there is visible on the cross section the 
, same concentrical disposition of the matter of the stem into 
; pith^ wood^ and bark^ and the same development of branches 
in the axilla of the leaves as in the stems of dicotyledonous 
j trees. The whole process of growth is_, however^ arrested in 
its first stages^ and the stem^ with its branches and fiowers_, 
I dies down to the ground and disappears from the earth^s 
' surface on the approach of winter. That the vegetable 
' machinery would continue in motion^ and simply stops in 
i consequence of the decreasing heat and light of the sun^ is 
! evident from the fact that plants which are annual and 
i herbaceous in temperate climates^ become ligneous and peren- 
nials in the tropics. Thus the castor- oil plant (Ricinus com- 
' munis), in Pennsylvania^ North America^ puts forth large 
! peltate-palmate leave s^ but is destroyed by the first frosts 
; of autumn. In tropical countries its stem is ligneous and 
persistent^ and it grows into a powerful and lofty tree. It is 
: the same with the Euphorbiaceae^ Labiat^^ Leguminosae,, 
i Boraginaceac, HypericaceaC;, Bubiaceae;, Yerbenaceae^ Poly- 
I gonaceae^ Compositae^ and a host of other plants which with us 
die annually^ and which in the tropics become trees^ elevating 
themselves majestically into the ah. The life of one generation 
of these plants is therefore now no longer limited to a year_, 
but prolonged for centuries. There is no authority^ then^ in 
nature for assigning any definite hmits to the number of gene- 
rations of trees^ but_, on the contrary^ every reasonable pre- 
sumption that the species of trees now living, and some of 
which we know to have been living for hundreds and even 
thousands of years, follow the same laws as herbaceous plants, 
and that each tree has been preceded by numerous generations ; 
