84 PROF. W. GAESTANG ON THE THEORY OF RECAPITULATION : 
necessarily to follow, however great the difBculties may be which are 
involved in their acceptance" (1885, ii. p. 32). But the "parallelism" of 
ontogenetic and phyletic sequences, which was incorporated by Haeekel 
in his "law," was noticed by many a "good physiologist" before him 
(c/. Meckel, Von Baer, L. Agassiz, &c.), and cannot disappear with his inter- 
pretation of it. Perhaps now we shall see its true extent and meaning moie 
clearly. Ontogeny proceeds through successive grades of differoitiaiion by 
which layers, tissues, organs, and parts together with ordinal, family, generic, 
and specific characters, are more or less successively established. As differ- 
entiation increases, the combination of layers, tissues, organs, and parts 
exhibited at successive stages resembles more or less distinctively the com- 
binations characteristic of successive grades of evolution represented in our 
schemes of phyletic classification. To that limited extent the ontogeny of 
a given animal iS an epitome of its phylogeny, and may be said, in the 
true sense of the word, to recapitulate phylogeny, i. e. to sum it up, 
recall the main phases of it. This is the parallelism observed by Meckel, 
Von Baer, and many others, expressed in evolutionary terms. It exists and 
is- undeniable. 
6. This parallelism exists because phylogeny is itself the creation of 
successive ontogenies, and ontogenies of necessit}' run parallel with one 
another from zygote to adult. For ontogeny is the expression of zygotic 
power, the function of zygotic structure ; and zygotic change involves no 
radical departure from the routine of ontogenetic method. One ontogeny is, 
in this sense, a modification of its predecessor. The ontogeny which first 
established the Coelenterate grade was the basis of a later ontogeny which 
established the Coelomate grade. The life-cycle was extended accordinglj', 
but never by the simple addition of a substantial unit or stage, distinctively 
Coelomate, to the final adult stage of a Coelenterate ontogeny. A house is 
not a cottage with an extra storey on the top. A house represents a higher 
grade in the evolution of a residence, but the whole building is altered — 
foundations, timbers, and roof — even if the bricks are the same. You may 
begin by building a cottage a little larger than its predecessoi-, cutting off 
an entrance passage from the parlour, and adding a back kitchen ; but when 
your ambition rises to an entrance hall, three reception rooms, two staircases,, 
and so on, you are forced to a mutation in your building plans which affects 
operations from the start. The ontogeny of a CJcelenterate adds, in a certain 
sense, on a simple diploblastic base, certain effective, workaday, adult 
features by which it copes with the conditions of its life ; but the replace- 
ment of these efl'ective characters by others suited to the more adventurous 
career of a Ccelomate {e.g. development of prehensile mouth instend of tentacles) 
involves their disappearance altogether ; and there remains of ('celenterate 
organisation only that diploblastic residuum of differentiation out of which 
the Coelomate may be economically and directly built up. Nor is the end of" 
