TO4 BOTANICAL GAZETTE . [AUGUST 
From this it follows that affinity is roughly proportional to resem- 
blance. Resemblance in one or a few points may mean a slight 
degree of affinity, or it may merely show a case of parallel develop- 
ment. Heterospory and the seed habit are excellent illustrations 
of the latter. If, on the contrary, two plants are very similar in 
all of their organs, the resemblance is usually considered unim- 
peachable evidence of close affinity. The classification of all our 
living plants is almost exclusively based on this principle. 
A corollary of this principle is that neither of two supposedly 
related plants must possess any organ or structure which cannot be 
reasonably derived from the homologous organ or structure of the 
assumed ancestor. 
2. The geological sequence should roughly conform to the pro- 
posed evolutionary sequence in the development of a modern group 
from an ancient one. It is obvious that if all the intermediate 
forms have been fossilized and all discovered, this agreement would 
be exact and complete. Such conditions doubtless never occur. 
Since evolution of related forms cannot be supposed to run exactly 
parallel in different lines, it follows that the discovered fossils from 
any given horizon might be expected to show one structure or 
organ ahead in one and another in another. An important corollary 
of this has frequently been insisted on by CoutTerR (14, 15, 16). 
One line of plants may run ahead along a certain line and remain 
practically stationary for ages in some other. For these reasons 
and because the fossil record is always very incomplete, inferences 
from geological sequence must always be subject to considerable 
oubt. 
3. Vestigial structures —By this is meant that anatomical char- 
acters that once were general through the entire plant are likely to be 
retained in certain supposedly primitive regions of the plant, such 
as the root, cotyledons, cone axes, and leaves. The use of this 
principle is attended with very considerable difficulties and may 
frequently lead to very erroneous conclusions. The difficulties lie 
in two assumptions that must be made inits application. First, we 
must assume that the stem structure of a paleozoic (let us say) plant 
was also present in the cone axis. Then when we find this same 
structure in the cone axis of a modern plant, we must again assume 
