` 1891.] The Evolution of the Circulatory Organs. 241 
- Thus we have traced, by easy and gradual steps, the complete 
evolution of the simple pulsating vessel of the Annelid unto 
the marvelously perfect organ of man, We have seen how the 
one pulsating tube has divided into two by a partial and then a 
perfect septum, into an auricle and a ventricle, and then have seen 
these cavities, by a partition more and more complete, separated 
into four distinct cavities. Yet all this is done with but slight 
alterations of preéxisting struc- 
ture, and without a link in the 
chain missing This is an argu-* 
ment approaching a demonstra- 
tion, and must appeal to all candid 
minds. To those who might 
object that even the slight 
changes could not be made with- 
out the destruction of the animal 
or species, I would instance the 
transformation of every tadpole 
into a frog. Surely no one will 
assume that tadpoles are changed 
now into a frog by any power 
save that residing in natural laws. 
Yet the changes are profound. 
The heart of a tadpole is practi- 
cally that of a fish, having one h heart e ie ian di Bnp 
auricle and one ventricle, and the “u7cl* in nin fhe gla d, he Brida f sid 
animal breathes by gills; yeta "89 P FTE 
frog has two auricles and a ventricle, and breathes by lungs. Here 
we see changes equivalent to the transformation of a fish into a 
reptile. And among reptiles we meet with hearts with everyjdegree 
of partition, until in the crocodile the heart is partitioned off very 
nearly the same as in birds and mammals. In the development 
of the heart in embryos of birds and mammals we find the organ 
passing through the conditions found permanently in lower forms. 
A distinguished comparative anatomist thus outlines the develop- 
ment of the embryo chick: “The first rudiments of the heart 
‘appear about the 27th hour, and is a mass of cells, of which the 
a 
