1891.] Sexual Immobility. 23 
generations—that rhythmic succession of sexual and asexual indi- 
viduals—which continues with perfect distinctness even into the 
order Compositz,—according to Luerssen, the highest family of 
plants. Why, now, there should be such a rhythm in plants 
and not in animals is a point to be explained, and very likely 
the explanation will throw light upon the origin and perpetu- 
ation of alternation in the vegetable kingdom. It is of no 
use to say, as Geddes does, quoted by Bowers, that alter- 
nation is “a rhythm between relatively anabolic and kata- 
bolic preponderance.” For why should such rhythm exist in 
plants and not in animals? In fine, there seems to be but one ex- 
planation readily entertained, and that is this: Adternation of gen- 
erations is a phenomenon conditioned upon individual immobility. 
It is readily discovered that in almost every species of animal 
free to move where it pleases alternation is not seen, while in many 
of the somewhat fixed and immobile alternation is apparent,—for 
example, in the Ccelenterates. And in the more mobile plants 
alternation is either indistinct (homologous) or scarcely apparent, 
while in the more immobile plants alternation becomes exceed- 
ingly distinct, and reaches its highest type in in the most immo- 
bile plants. Now the sexual act, being conditioned upon the 
approach to each other of the gametangia or gametes themselves, 
is accomplished with greater and greater difficulty the more com- 
plete the immobility of the sexual individual. In animals immo- 
bility is at its maximum; in the higher plants—notably most 
Angiosperms—it is at its minimum. In other words, the sexual 
act is easiest of accomplishment in animals, speaking generally, 
and most difficult in the higher plants; consequently the egg, 
in animals, is easily produced, but in a plant of the Archegoniate 
series an egg is produced with great difficulty. In spite of the 
high gonotropic specializations in other plants, probably not a 
millionth part of the sexual individuals produced ever accomplish 
their end. For this reason the egg of an animal is, on the whole, 
a cheap product; the egg of an Archegoniate plant is an expen- 
stve product. ; 
From the considerations outlined above we have no difficulty in 
perceiving the physiological conditions which lead to the appear- 
