850 REPORT—1890. 
bination 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, 7.c.,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-fertilisation, then it may be pointed out in reply that 
sexual reproduction is the characteristic and essential mode of multiplication 
among Metazoa: that it occurs in all Metazoa, and that when asexual reproduc- 
tion, as by budding, &c., occurs, this merely alternates with the sexual process 
which, sooner or later, becomes essential. 
If the fundamental importance of sexual reproduction to the welfare of the 
speciesbe granted, and if it be further admitted that Metazoa are descended from Pro- 
tozoa, then we see that there is really a constraining force of a most powerful nature 
compelling every animal to commence its life history in the unicellular condition, 
the only condition in which the advantage of cross-fertilisation can be obtained ; 
z.e., constraining every animal to begin its development at its earliest ancestral 
stage, at the very bottom of its genealogical tree. 
On this view the actual development of any animal is strictly limited at both 
ends: it must commence as an egg, and it must end in the likeness of the parent. 
The problem of recapitulation becomes thereby greatly narrowed ; all that remains 
being to explain why the intermediate stages in the actual development should 
repeat the intermediate stages of the ancestral history. 
Although narrowed in this way, the problem still remains one of extreme 
difficulty. 
It is a consequence of the Theory of Natural Selection that identity of structure 
involves community of descent: a given result can only be arrived at through a 
given sequence of events: the same morphological goal cannot be reached by two 
independent paths, A negro and a white man have had common ancestors in the 
past; and it is through the long-continued action of selection and environment 
that the two types have been gradually evolved. You cannot turn a white man 
into a negro merely by sending him to live in Africa: to create a negro the whole 
ancestral history would have to be repeated; and it may be that it is for the same 
reason that the embryo must repeat or recapitulate its ancestral history in order to 
reach the adult goal. 
Tam not sure that we can at present get much further; but the above con- 
siderations give opportunity for brief notice of what is perhaps the most note- 
worthy of recent embryological papers, Kleinenberg’s remarkable monograph on 
Lopadorhynchus. 
Kleinenberg directs special attention to what is known to evolutionists as the 
difficulty with regard to the origin of new organs, which is to the effect that 
although natural selection is competent to account for any amount of modification 
in an organ after it has attained a certain size, and become of functional import- 
ance, yet that it cannot account for the earliest stages in the formation of an organ 
before it has become large enough or sufficiently developed to be of real use. The 
difficulty is a serious one; it is carefully considered by Mr. Darwin, and met 
completely in certain cases; but, as Kleinenberg correctly states, no general 
explanation has been offered with regard to such instances. 
As such general explanation Kleinenberg proposes his theory of the develop- 
ment of organs by substitution. He points out that any modification of an organ 
a 
