THE ORIGIN OF ANGIOSPERMS. 35 
The Law of Corresponding Stages in Evolution. 
If we study the stages in evolution reached by the different organs of a 
seed-plant at any one period, we shall find that they are dissimilar. Some 
are obviously more highly evolved than others. Corresponding stages in the 
evolution of the various members of a seed-plant are not contemporaneous 
in point of time. Conversely, at any one period in geological time, one organ 
or set of organs will be found to have reached a far higher stage of evolution 
than another. 
The study of fossil botany has afforded numerous instances of. the truth of 
this principle. The foliage of the Pteridospermes, as also the habit of the 
stem , 1s essentially fern-like, though the female organ is а seed. "The male 
organs ( Crossotheca) of Lyginodendron are obviously far less highly evolved 
than the female (Lagenostoma). "The former is essentially a. simple fern-like 
fructification, the latter a highly evolved seed. 
Or, again, to turn to the Bennettitez, the Mesozoic descendants of the 
Pteridospermesz, the mierosporophylls are still essentially fern-like fronds, 
while the megasporophylls are of an extremely advanced type. The trunk 
habit of these plants also shows scarcely any modification as compared 
with that of a Paleozoic tree-fern. 
Or to turn to recent plants. The fern-like foliage of the Cycad Stangeria 
is associated with highly-evolved strobili. Again, the female sporophyll of 
Cycas is more primitive than the male, and its fern-like origin is still 
traceable. 
Other instances might be quoted *, but the above may suffice. It follows 
from this law that there was never in existence such a plant as a really 
primitive Angiosperm, in the sense that all its organs were equally of a 
primitive nature. On the contrary, the earliest Angiosperms were no doubt 
characterised by possessing some members much more highly evolved than 
others. There is reason to believe that the Angiosperms were derived from 
other seed-plants, but that the seed, in itself a highly evolved structure, 
originated at а much more remote period. 
We believe that the application of this principle will be found to be of 
great service in such considerations as the present. At a later stage (p. 70) 
we propose to demonstrate by its aid the probability that the earlier 
Angiosperms still retained the megaphyllous foliage of their ancestors ; а 
supposition which will explain several puzzling facts. 
Homoplasy. 
It is well recognised that one of the great difficulties to be faced in 
attempting to trace the phylogeny of living groups lies in the tendency to 
error, arising from laying stress on certain features as indicative of close 
* See Wieland (1906) p. 244, who has also discussed this law. 
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