g., as the buds of the
strawberry sucker tend to become typical strawberry plants. In the case
of the progressive evolution of a species, the developmental
potentiality of B becomes of a higher and higher order. In
retrogressive evolution, the contrary would be the case. The phenomena
of atavism seem to show that retrogressive evolution that is, the
return of a species to one or other of its earlier forms, is a
possibility to be reckoned with. The simplification of structure,
which is so common in the parasitic members of a group, however, does
not properly come under this head. The worm-like, limbless Lernoea has
no resemblance to any of the stages of development of the many-limbed
active animals of the group to which it belongs. [89] Note 2 (p. 49).
Heracleitus says,[Greek phrase Potamo gar ouk esti dis embenai to suto]
but, to be strictly accurate, the river remains, though the water of
which it is composed changes--just as a man retains his identity
though the whole substance of his body is constantly shifting.
This is put very well by Seneca (Ep. lvii. i. 20, Ed. Ruhkopf):
"Corpora nostra rapiuntur fluminum more, quidquid vides currit cum
tempore; nihil ex his quae videmus manet. Ego ipse dum loquor mutari
ista, mutatus sum. Hoc est quod ait Heraclitus 'In idem flumen bis non
descendimus.
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