218 PROF. HUXLEY ON THE AGAMIC REPRODUCTION 
Truly we may say, with Degeer (J. c. p. 129), * Les Pucerons sont des insectes bien 
capables de déranger tout système formé de génération, et de mettre en déroute tous ceux 
qui s'efforcent d'expliquer ce mystère de la nature.” 
But the question may be asked : if the “spermatic force” be a myth, what és the cause 
of the phenomena? Considering that the groundwork of modern physiology is not a 
score of years old, I do not think the confession of our inability to answer that question 
at present is any opprobrium to science. 
When we know why, in a mass of tissue of identical structure throughout, one part 
becomes a brain, and another a heart, and a third a liver—when we can answer these 
every-day questions of the sphinx, we may attempt her more difficult riddles without 
running too great a risk of being devoured. 
At the present time it seems to me well nigh hopeless to look for an explanation of 
these phenomena. Some such classification of them, however, as will indicate their 
analogies with other vital manifestations, may fairly be attempted, and, when successfully 
carried out, will prove the first step towards an explanation. 
$ 7. Classification of the Phenomena of Agamogenesis. 
It does not seem to be very difficult to effect such a classification. In the course of the 
development of the total product of a single impregnated ovum (which, with Dr. Carpenter, 
I regard as the zoological individual), one of two things may occur: either all the living 
products may remain in connexion with one another, or they may become separated from 
one another. The former case I term Continuous, the latter Discontinuous Development. 
In continuous development, the size may increase, the form and texture remaining 
unchanged—constituting simple growth; or, the size remaining unchanged, the form 
and texture may alter—constituting simple metamorphosis; or the two processes may be 
combined, as in all those changes which we term gemmation, without separation from 
the parent. | 
Discontinuous development differs from continuous only in this, that the products of 
the growth and metamorphosis of the embryo become separated into two or more portions, 
which when they retain their vitality independently are termed * zooids.” | 
When the produced *zooid” is capable of development into an independent organism 
without the influence of an act of conjugation with another zooid, I term the process 
agamogenesis. The producing zooid may be devoid of sexual organs, as in the Salpe, 
many Hydrozoa, many Trematoda—in fact, in the great majority of cases of agamogenesis. 
I term the first producing zooid of the individual the protozooid ; the produced zooids, 
deuterozooids. In some cases the deuterozooids acquire sexual organs, and give rise to 
ova and spermatozoa ; but in others they produce new zooids : thus broods of tritozooids, 
&e., will be produced. When the producing or protozooid possesses no sexual organs, 
I think Prof. Owen’s term of *metagenesis” might well be applied to the kind of 
agamogenesis ; but where the protozooid possesses sexual organs, and its buds have all 
the histological characters of ova, then the process may fairly enough be termed parthe- 
nogenesis. 
