132 THE AMERICAN WATER WEED. 
All these causes influence, but no one of them individually 
fully accounts for the rapid spread and present abundance of 
Anacharis; which has also been further aided by the aquatic 
nature of the plant, and the consequent readiness with which 
floating vegetative fragments are conveyed over the whole water 
system of the country ; or by adhering to the feet and plumage 
of water fowl are carried to isolated pieces of water. 
Somewhat comparable instances of increased fecundity of 
introduced plants and animals are frequent. As, for example, 
the Scotch Thistle, Castor Oil Plant, and common Sparrow, in 
Australia ; Xanthium spinosum in South Africa ; European Arti- 
chokes in South America; the common Asiatic Cockroach in 
Great Britain; the Norway Rat in our farmyards; Drezssena 
polymorpha in our canals and rivers. But none of these well- 
known examples are exactly analogous with Anacharis because 
they are all reproduced sexually, whereas it is invariably in- 
creased asexually. And I have sought in vain for any plant or 
animal wherewith strictly to compare Anacharis as to its peculiar 
mode of spreading and rate of increase. 
As to its decrease, I have no doubt whatever that it is no 
longer increasing. In many places it continues to grow with 
unabated vigour ; in others it just maintains its ground ; in some 
others it is manifestly less in quantity; and from some ponds, 
ditches, and streams where it was quite recently abundant, it has 
now entirely disappeared. Taking it altogether, it appears that 
its maximum abundance is past, and that it is certainly on the 
wane in our own district and also elsewhere. This decrease I 
believe to be chiefly due to a cause which at first is most powerful 
in promoting its increase, viz., the non-reproduction by seed. 
Plants which depend for their continuance upon non-sexual 
reproduction gradually become more and more enfeebled in con- 
stitution. They are certainly not so hardy as seedling plants, 
nor able so vigorously to contend in the struggle for existence. 
The lengthened cycle of phenomenal activity which is presented 
to us by Anacharis, has its exact analogue in the much more 
brief life histories of some of the lower forms of vegetable life ; 
and may not the same reasons apply to the still dimly understood 
phenomena connected with rapid spread at intervals of disease 
germs? May they not, under particular conditions, develop a 
generation with excessively active powers, which shall multiply 
for a time asexually, until its vigour be in part or completely 
exhausted, when it either perishes or reverts to its normal com- 
paratively harmless condition, and so remain until the recurrence 
of certain external conditions shall once more excite it into 
abnormal activity. 
The study and consideration of such phenomena suggests the 
idea that the vitality of an organism progressively diminishes 
throughout life; commencing at birth with the maximum, and 
gradually expending it in the various so-called vital functions, 
