431 SUMMARY OF CURRENT RESEARCHES RELATING TO 
from a duck embryo on the second day by the inspection of a 
single transverse section through the trunk ; Peripatus capensis and 
P. Bcdfouri are so alike when adult, that if it had not been for their 
embryonic differences, Mr. Sedgwick would not have instituted the 
second species. Indeed, it may be concluded that a species is distinctly 
distinguishable from its allies from the very earliest stages all through 
the development, although the 'embryonic differences do not necessarily 
implicate the same organs as do the adult differences. 
With regard to the significance of ancestral rudiments in embryonic 
development, Mr. Sedgwick points out that the assumption that the 
repetition of ancestral characters in embryogeny is the intelligible rule is 
not warranted by the fact that in the vast majority of ontogenies there 
are no phylogenetic traces, nor by the consideration that a number of 
important organs, such as teeth in birds, limbs in snakes, gill-clefts in 
fishes, have recently disappeared without leaving a trace in ontogeny. 
The balance of evidence appears to Mr. Sedgwick to point most 
clearly to the fact that the tendency in embryonic development is to 
directness and abbreviation, and to the omission of ancestral stages of 
structure. The fact that some organs are represented by vestiges, while 
others leave no trace, is perhaps to be explained by the differences be- 
tween the embryonic and the larval modes of development ; modes 
which have generally not been properly distinguished by naturalists 
who have written on the subject. 
In embryos the organs are for the most part functionless, and with- 
out relation to the maintenance of life, so that there is nothing to 
counteract the tendency to the appearance of a variation at all stages in 
the life of an organ. In larvae, on the other hand, the organs are func- 
tional, and if a variation of an organ at one stage is injurious to the 
same organ at a previous or a subsequent stage, it will be eliminated at 
the stages at which it is injurious. 
The conclusion to which the author arrives is that, whereas larval 
development must retain traces of ancestral stages of structure, because 
they are built out of ancestral stages, embryonic development need not 
necessarily do so, and very often does not ; embryonic development, in 
so far as it is a record at all, is a record of structural features of previous 
larval stages. 
Mr. Sedgwick points out that the principles which he sets forth ex- 
plain why it is that in higher animals it is the early stages of develop- 
ment which have the greatest interest for us, the later stages having been 
added at a time when, as now, the immature stages of free life were but 
little marked, and consequently there was but little chance of the incor- 
poration of any ancestral features in the embryonic development. They 
help us, further, to understand why the most interesting of the ancestral 
embryonic features were related to the passage from the aquatic to the 
terrestrial condition, because when this took place in phylogeny there 
must have been a most pronounced aquatic larval stage such as we find 
to-day in Amphibia. 
Effect of External Conditions on Development.* — The pith of 
Prof. A. Weismann’s Komanes lecture appears to lie in the doctrine that 
even when “ to all appearance external influences have had direct action 
in causing purposeful modifications, a more careful examination will 
* Nature, 1. (1894) p. 31. 
