284 Strasburger . — The Pejdodic Reduction of 
the general law which determines the phylogenetic disappear- 
ance of organs which have become useless, the vegetative 
parts of the sexual generation became more and more reduced, 
until little was left but the reproductive organs themselves : 
hence the progressive reduction in the prothallium from the 
Ferns up to the Phanerogams. This reduction culminated in 
the complete loss of independent existence by the sexual 
generation, because it had ceased to be able to nourish itself 
independently, and its becoming enclosed by the asexual 
generation. In consequence of this enclosure of the sexual 
in the asexual generation, the advantageous rapid multiplica- 
tion of individuals which the latter originally effected was 
lost : in order to compensate for this loss, a large number 
of seeds were produced in the Phanerogams in place of the 
numerous spores of the Cryptogams ; that is, multiplication is 
effected now by the products of fertilization instead of by 
asexual spores. 
Alternation of generations is absolutely necessary only in 
those groups of plants in which the fertilized ovum gives rise 
to the asexual generation, and the asexual spore to the sexual 
generation. In all such plants the asexual generation is the 
product of a sexual act. It is important to draw attention to 
this fact, since it forms the basis of the views which will be 
subsequently expounded. 
From what has been stated in the foregoing paragraphs, it 
it is clear that throughout the Plant-Kingdom (as far as 
sexuality is present), sexual differentiation was preceded by 
asexuality. On the other hand, it must be clearly appre- 
hended that in all those divisions of the Plant-Kingdom in 
which a true alternation of generations obtains — that is, a 
necessary alternation of a sexual with an asexual generation — 
as in the Muscineae, Vascular Cryptogams, and Phanerogams, 
the sexual generation is to be regarded as the older and 
as having arisen from an asexual form. Similarly, compara- 
tive investigation teaches that the second generation, developed 
from the product of a sexual act, must be phylogenetically the 
younger. The gradual development of this generation from 
