a 
TRANSACTIONS OF SECTION D. 835 
Amount of modification is an ambiguous term, and covers two distinct varieties 
of evolution: 
(i.) Increase in differentiation of organs fulfilling the main functions (nervous, 
muscular, circulatory, and reproductive organs, for instance), correlated with 
greater intensity of metabolism. 
Gi.) Modification of shape, size, and external organs. 
(i.) is regarded by most zoologists as the essence of progressive evolution. 
The lesser value assigned to (ii.) justifies the separation of the Thylacine 
and Dog. 
The answer to the second question is, so far as can be inferred from data laid 
down above, in the negative. 
Hence it is not legitimate to assume that Vertebrata are directly descended 
from Balanoglossus or even Amphioxus. 
On the main question as to the criteria of primitive and degenerate cha- 
racters. Primitive structures are synthetic in nature; they either serve to link 
together different groups, as the flat foot of Nucula connects Gastropods and 
Lamellibranchs, or different organs, as the ccelom of the lower Annelids and of 
Brachiopods unites the functions of the renal and reproductive organs; for new 
organs have not arisen de novo from functionless rudiments, but by the modifica- 
tion of pre-existing organs. 
Degenerate structures do not recall structures of other groups, and their con- 
dition does not correspond to the evolutionary level deducible from the condition 
of the other organs of the body. 
Example: Rudimentary limbs of certain Urodeles. 
8. One of the most vexed questions in zoology is the value to be attached to 
ontogeny as a record of phylogeny. Some have denied that it has any such 
value, but cases exist where the phylogenetic value is simply undeniable. 
It is highly improbable that ontogeny is a process of an essentially different 
nature in different cases; therefore there is probably a phylogenetic element in 
all ontogeny. 
Many features in embryology are, as all admit, secondary. 
The key to the puzzle is that the embryo is a modified larva, and that the 
larva recapitulates not primarily ancestral structure but— 
(a) Ancestral habits. 
(6) Ancestral level of differentiation of functions, and ancestral structure so 
far as is demanded by these conditions. 
4, In relation to the question as to how far homoplasy interferes with the 
conclusions we are accustomed to base on similarity of structure, it must be 
admitted that parallel development has not only taken place in widely separated 
groups, where there is no danger of confusion, but again and again in narrow 
circles of affinity; the researches of modern systematists seem to show that it is 
the normal thing. Instances of this, Arion and Limax amongst Pulmonata, &c. 
Criticism of the conception of identity of ancestry. 
We do not mean that animals belonging to different families are ultimately 
descended from the same pair. We mean only from ancestors so similar as to have 
been able to pair with one another, or in other words belonging to same species. 
Species are, however, often separated by trivial marks, so far as we can see, 
of a non-adaptive character. It is a gratuitous assumption that similarity in 
broad outlines of structure which are adaptive indicates descent from same 
species. 
Closely allied species exposed to same environmental influence would undergo 
the same change; descent from same species is only the extreme term in a series 
in which there is a gradual passage from what would be called homology to 
undeniable homoplasy. Structural resemblance indicates not primarily identity 
of ancestry, but similarity of past environment; and there may be all degrees 
‘in this similarity, both in extent and duration. 
A conclusion like this is tacitly admitted by systematists who make the basis 
of their system minute and apparently unimportant peculiarities of external form, 
