28 
COMPARISON 
tendrils, thorns, and leaves, agreeing in position, mode of 
origin, & c. upon closely related plants, are mutually homo- 
logous. During the descent from Y change of structure 
and function has gone on in at least two of the lines, as 
there are now three types of structure and function. As C 
and D have leaves, the probability is that Khad leaves also, 
and that the organs of A and B are 4 modified ’ leaves, but 
to confirm this supposition we must examine other nearly 
related forms E , P, G , H, & c. If all these have leaves, 
then probably Z, the common ancestor of these and of 
A, B, C , D , had also leaves. Z may be the same as Y, 
but probably is older ; in any case, however, the supposition 
that Y had leaves is strengthened. The points in which 
A, B, C, D, E , &c. all agree will in general be those handed 
down from Z. 
Evidently, then, the more that the plants examined 
differ, the further back will be the ancestral form, the less 
can we infer about that form, and the greater will be the 
difficulty of tracing the phylogenies and the homologies. 
We must also be careful in using such expressions as 
‘modified leaf.’ 
Position, Mode of Origin, and Internal Structure 
are characters of very great importance in morphological 
comparisons, and are largely used in practice. Homologous 
organs often agree in these characters, if in no others, 
e.g. ‘ shoot-branches ’ usually arise in leaf-axils, exogenously, 
and have * shoot ’ anatomy, though they may actually be 
tendrils or thorns. 
Development of the individual organ from the 
embryonic condition, i.e. its ontogeny, often affords useful 
aid in tracing phylogenies and homologies. Such investi- 
gations need the aid of the microscope, must be comparative, 
and their results must be tested by those of other methods. 
Speaking broadly, allowing for exceptions and variation in 
detail, it may be said that the metamorphoses of the ontogeny 
tend to recapitulate those of the phylogeny. The more 
recent the latter, the. more clearly they will show in the 
former, while the earlier stages will tend to run rapidly 
into one another. Suppose two organs of common 
descent to have passed through phylogenetic changes 
represented by ABCDEFGHKL and A B CDEMNOPQ, 
