EVOLUTION BY ACCELERATION AND RETARDATION. 167 



given in figures 193 E and 194, to the free swimming Copepoda. 

 Finally, he says a few words on the theory of evolution, and 

 remarks "there is no doubt that, if a solution of the questions 

 arising concerning the genealogical relations of ditferent animals 

 among themselves is possible, comparative embryology will af- 

 ford the first and truest principles." He modestly suggests that 

 the facts presented in his paper will widen our views on the 

 genetic relations of the insects to other animals, and refers to 

 the opinion first expressed by Fritz Miiller (Fiir Darwin, p. 91), 

 and endorsed by Hseckel in his "Generelle Morphologic," that 

 we must seek for the ancestors of insects and Arachnida in the 

 Zoea form of Crustacea. He cautiously remarks, however, that 

 " the embryos and larvae observed by me in the egg-parasites 

 open up a new and wide field for a whole series of such consid- 

 erations ; but I will suppress them, since I am firmly convinced 

 that a theory, which I build up to-day, can easily be destroyed 

 with some few facts which I learn to-morrow. Since compara- 

 tive embryology as a science does not yet exist, so do I think 

 that all genetic theories are too premature, and without a strong 

 scientific foundation." 



The writer is perhaps less cautious, but he cannot refrain 

 from making some reflections suggested by the remarkable dis- 

 coveries of Ganiu. In the first place, these facts bear strongly 

 on the theory of evolution by "acceleration and retardation." 

 In the history of these early larval stages we see a remarkable 

 acceleration in the growth of the embryo. A simple sac of 

 unorganized cells, "with a half-made intestine, so to speak, is 

 hatched, and made to perform ihe duty of an ordinary, quite 

 highly organized larva. Even the formation of the "primitive 

 band," usually the first indication of the organization of the 

 germ, is postponed to a comparatively late period in larval life. 

 The different anatomical systems, i.e., the heart with its vessels, 

 the nervous system and the respiratory system (tracheae), appear 

 at longer or shorter intervals, while in one genus the trachea; 

 are not developed at all. Thus some portions of the animal are 

 accelerated in their development more than others, while others 

 are retarded, and in some species certain organs are not devel- 

 oped at all. ]\Ieanwhile all live in a fluid medium, with much 

 the same habits, and surrounded with quite similar physical 

 conditions. 



The highest degree of acceleration is seen in the reproductive 



