626 ZOOLOGY SECT. 



which the animal belongs has passed in the course of its evolution 

 from lower forms. This supposition the " biogenetic law," or 

 " recapitulation theory," as it is termed though it cannot be 

 accepted without great modifications and reservations, yet covers 

 a number of facts which distinctly demand a process of evolution 

 for their explanation. 1 



The phenomenon of retrograde metamorphosis observable in 

 many animals, for the most part parasitic in the adult condition, 

 also affords evidence in favour of evolution. It would be difficult 

 to give any other explanation than that afforded by a theory of 

 descent, of the life-history of such animals as Sacculina (Vol. I., 

 p. 583), the parasitic Copepoda (p. 583), or the Ascidians (Vol. II., 

 p. 35). The relatively high organisation of the larva of Sacculina, 

 for example, with its well-marked Crustacean features, can only be 

 explained on the supposition that the shapeless, unsegmented 

 adult has been derived by a process of retrograde development from 

 more normally constructed ancestors. 



Most Birds and Mammals, and many animals of lower groups, 

 exhibit a more or less strongly marked sexual dimorphism, the 

 males differing from the females in various other respects besides 

 the character of the sexual organs. Such differences can only be 

 explained on the supposition that they are the result of a gradual 

 process of modification brought about in accordance with the 

 more special adaptation of each sex to its special functions. 



Palaeontological Evidence. A second body of evidence in 

 favour of a theory of evolution comes from the side of Palaeontology. 

 It might, perhaps, on first considering the subject, be supposed 

 that, had a process of evolution taken place, we ought to be able 

 to find in the rocks belonging to the various geological formations 

 a complete series of animal- and plant-remains representing all 

 the stages in the evolution of the highest forms from the lowest. 

 Beginning with those strata in which evidence of life first appears, 

 we ought, it might be supposed, to be able to trace upwards, 

 through all the series of fossil-bearing strata, continuous, unbroken 

 lines of descent showing the gradual evolution of all the various 

 forms of plant- and animal-life. But such a supposition would 

 leave out of account the extreme incompleteness of the record of 



1 As an instance of the difficulties in the way of the acceptance of the 

 biogenetic law as such, it may be pointed out here that the ovum is by no 

 means equivalent to the simple cell with which the phylogenetic series must 

 be supposed to have begun. On the contrary, the ovum of one of the higher 

 animals must be an extremely complex structure, and in reality widely 

 different from the Protozoan which, according to the biogenetic law, should 

 be its prototype. The ovum of the higher animal is, it is true, a single cell ; 

 but it is a cell which comprises potentially within itself the entire complex 

 adult organism, and is thus essentially an entirely 'different thing from the 

 unicellular Protozoan. The same holds good of later developmental stages : 

 they may resemble the adult condition of lower groups ; but they differ from 

 the latter in the same way as the ovum differs from the Protozoan. 



