XXI.] EMBRYOLOGY. 271 



1. Water-breathing Vertebrates (fishes) producing their Fishes. 

 like. 



2. Water-breathing Vertebrates (tadpoles) developing Batra- 

 into air-breathers, which again produce water-breathing ° 

 tadpoles from their eggs. 



3. Air-breathing Vertebrates (the land salamander Air- 

 among Batrachians and all the higher vertebrate classes) verte- ^ 

 producing their like. brates. 



I believe these three form a succession by descent in 

 the order enu.meTated ; and I believe all the classes of 

 air-breathing Vertebrates have been in this way descended Descent of 



. . the latter 



from fishes, of which descent their embryos still bear the ^^^ 

 marks in the "branchial slits" of their arteries. fishes. 



The existence of similar series in other parts of the Series. 

 animal kingdom is to be explained in the same way. 

 Thus we find this series : — 



1. Worms producing worms. Worms. 



2. Worm-like larvae developing into hexapod insects. Insects 

 which again produce worm-like larvae from their eggs. from °^"^ 



3. Hexapod insects (as Aphis) producing hexapod insects, worm-like 

 which after they leave the egg undergo little or no meta- 

 morphosis except the acquisition of wings. No insect. Insects 



. . .1 . „ . T directl}' 



however, acquu-es wmgs until some time after it leaves developed. 

 the egg. 



Among the higher, or malacostracan Crustacea, we find Crustacean 

 quite as remarkable a series, as follows : — 



1. Nauplius producing its like. This, I believe, is Nauplius. 

 not known by direct evidence to exist or to have ever 

 existed ; but we may infer its former existence from the 



fact that " forms wonderfully distinct from each other, as 

 the suctorial parasites, Cirrhipedes, Entomostraca, and 

 even [a few of] the Malacostraca, appear in theu" first 

 larval state under a similar nauplius form." ^ From such a 

 Nauplius all those forms have probably been descended. 



2. Nauplius developing into zoea, thence into mysis, 



and thence into the mature malacostracan form of Penoeus, Penceus. 

 or a genus allied thereto, which again produces Nauplii 



1 Darwin's Origin of Species, p. 531. Darwin agrees with Fritz Miiller 

 in drawing these inferences. 



