Aug., 1891 . 
ANIMAL PEDIGREES. 
177 
A further consideration of importance is that recapitula¬ 
tion is not seen in all forms of development, but only in 
sexual development ; or, at least, only in development from 
the egg. In the several forms of asexual development, of 
which budding is the most frequent and most familiar, there 
is no repetition of ancestral phases ; neither is there in cases 
of regeneration of lost parts, such as the tentacle of a snail, 
the arm of a starfish, or the tail of a lizard; in such 
regeneration it is not a larval tentacle, or arm, or tail, that 
is produced, but an adult one. 
The most striking point about the development of the 
higher animals is that they all alike commence as eggs. 
Looking more closely at the egg and the conditions of its 
development, two facts impress us as of special importance ; 
first, the egg is a single cell, and therefore represents 
morphologically the Protozoan, or earliest ancestral phase ; 
secondly, the egg, before it can develop, must be fertilised 
by a spermatozoon, just as the stimulus of fertilisation by the 
pollen grain is necessary before the ovum of a plant will 
commence to develop into the plant-embryo. 
The advantage of cross-fertilisation in increasing the 
vigour of the offspring is well known, and in plants devices 
of the most varied and even extraordinary kind are adopted 
to ensure that such cross-fertilisation occurs. The essence of 
the act of cross-fertilisation, which is already established 
among Protozoa, consists in combination of the nuclei of two 
cells, male and female, derived from different individuals. 
The nature of the process is of such a kind that two 
individual cells are alone concerned in it; and it may, I 
think, be reasonably argued that the reason why animals 
commence their existence as eggs, i.e ., as single cells, is 
because it is in this way only that the advantage of cross¬ 
fertilisation can be secured, an advantage admittedly of the 
greatest importance, and to secure which natural selection 
would operate powerfully, 
The occurrence of parthenogenesis, either occasionally or 
normally, in certain groups is not, I think, a serious objection 
to this view. There are very strong reasons for holding that 
parthenogenetic development is a modified form, derived from 
the sexual method. Moreover, the view advanced above does 
not require that cross-fertilisation should be essential to 
individual development, but merely that it should be in the 
highest degree advantageous to the species, and hence leaves 
room for the occurrence, exceptionally, of parthenogenetic 
development. 
If it be objected that this is laying too much stress on 
sexual reproduction, and on the advantage of cross-fertilisa- 
