PARALLELISM. 207 



not used their results with proper caution, have been 

 frequently led to incorrect and even absurd results. 

 The errors of this class of biologists are mainly due to 

 their ignorance of species in the adult state, and their 

 ignorance of systematic biology or taxonomy. They 

 profess to regard this branch of the science as only 

 suitable for beginners, and as comparatively unim- 

 portant, as compared with their own ; yet one might 

 as well attempt the study of philology without a knowl- 

 edge of alphabets, as to study phylogeny without the 

 knowledge of natural taxonomy. The correct discrim- 

 ination of species, genera, etc., imposes much greater 

 burdens on the faculty of judgment, than does any- 

 thing to be found in any science which includes obser- 

 vation and record only. But Mr. Hurst's statement is 

 somewhat overdrawn, and he does not give embryolo- 

 gists the credit which is due to their theory of recapitu- 

 lation. I think he will find the following, which I wrote 

 in 1872^ to be a correct statement of the facts, and a 

 fair induction as to principles. 



"The smaller the number of structural characters 

 which separate two species when adult, the more 

 nearly will the less complete of the series be identical 

 with an incomplete stage of the higher species. As 

 we compare species which are more and more differ- 

 ent, the more necessarily must we confine the asser- 

 tion of parallelism to single parts of the animals, and 

 less to the whole animal. When we reach species as 

 far removed as man and a shark, which are separated 

 by the extent of the series of vertebrated animals, we 

 can only say that the infant man is identical in its nu- 

 merous origins of the arteries from the heart, and in 

 the cartilaginous skeletal tissue, with the class of 



\Penn Monthly, 1872. Origin of the Fittest, 1887, p. 8. 



