The Theory of Evolution 59 
The tadpole stage is a true snail; it has gills which hang 
free at the sides of the body as is the case in Unio pictorum. 
It has even a byssus, as in Mytilus, in order to cling to the 
grass. The tail is nothing else than the foot of the snail. 
The metamorphosis of an insect is a repetition of the whole 
class, scolopendra, oniscus, julus, spider, crab.” 
Walther, in 1808, said: “ The human foetus passes through 
its metamorphosis in the cavity of the uterus in such a way 
that it repeats all classes of animals, but, remaining perma- 
nently in none, develops more and more into the innate 
human form. First the embryo has the form of a worm. 
It reaches the insect stage just before its metamorphosis. 
The origin of the liver, the appearance of the different secre- 
tions, etc., show clearly an advance from the class of the 
worm into that of the mollusk.” 
Meckel first in 1808, again in 1811, and more fully in 1821 
made much more definite comparisons between the embryos 
of higher forms and the adult stages of lower groups. He 
held that the embryo of higher forms, before reaching its com- 
plete development, passes through many stages that corre- 
spond to those at which the lower animals appear to be 
checked through their whole life. In fact the embryos of 
higher animals, the mammals, and especially man, correspond 
in the form of their organs, in their number, position, and 
proportionate size to those of the animals standing below 
them. The skin is at first, and for a considerable period of 
embryonic life, soft, smooth, hairless, as in the zoophytes, 
medusz, many worms, mollusks, fishes, and even in the 
lower amphibians. Then comes a period in which it becomes 
thicker and hairy, when it corresponds to the skin of the 
higher animals. It should be especially noted here, that the 
foetus of the negro is more hairy than that of the European. 
The muscular system of the embryo, owing to its lack of 
union in the ventral wall, corresponds to the muscles of the 
shelled, headless mollusks, whose mantle is open in the same 
