112 Darwin, and after Darwin. 
colonies of Protozoa. And its marvellous development 
in the Metazoa appears ultimately to depend upon the 
highly specialized character of germ-cells. Even in 
cases where multicellular organisms are capable of re- 
producing their kind without the need of any preceding 
process of fertilization (parthenogenesis), and even in 
the still more numerous cases where complete or- 
ganisms are budded forth from any part of their parent 
organism (gemmation, Fig. 28), there is now very good 
reason to conclude that these powers of a-sexual 
reproduction on the part of multicellular organisms 
are all ultimately due to the specialized character of 
their germ-cells. For in all these cases the tissues of 
the parent, from which the budding takes place, were 
ultimately derived from germ-cells—no matter how 
many generations of budded organisms may have 
intervened. And that propagation by budding, &c., 
in multicellular organisms is thus ultimately due to 
their propagation by sexual methods, seems to be 
further shown by certain facts which will have to be 
discussed at some length in my next volume. Here, 
therefore, I will mention only one of them—and this 
because it furnishes what appears to be another 
important distinction between the Protozoa and the 
Metazoa. 
In nearly all cases where a Protozoén multiplies 
itself by fission, the process begins by a simple 
division of the nucleus. But when a Metazo@n is de- 
veloped from a germ-cell, although the process likewise 
begins by a division of the nucleus, this division is not 
asimple or direct one ; on the contrary, it is inaugurated 
by a series of processes going on within the nucleus, 
which are so enormously complex, and withal so 
