610 ZOOLOGY SECT. 



course of its evolution from lower forms. This supposition the 

 " biogenetic law," or " recapitulation theory," as it is termed we 

 shall return to later. 



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. 553), the parasitic Copepoda (p. 552), or the Ascidians (Vol. II. 

 p. 27). 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 di- 

 morphism, the males differing from the females in various other 

 respects besides the character of the sexual organs. Such differ- 

 ences can only be explained by 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 Palae- 

 ontology. 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 evolu- 

 tion of all the various forms of plant -and animal life. But such 

 a supposition would leave out of account the extreme incom- 

 pleteness of the history of life on the globe which is preserved to us 

 in the rocks. In the first place, there are many groups of animals 

 and plants which, owing to the absence of any hard supporting 

 parts, are incapable of leaving any recognisable trace of their 

 former existence in the form of fossils. Again, even in the 

 case of such as have such hard parts, the conditions necessary for 

 their preservation in deposits destined to be converted into rock 

 cannot be of very frequent occurrence ; and many forms might fail 

 to be preserved simply owing to the non-occurrence of such 

 conditions. In the case of land animals, such as Mammals or 

 Keptiles, for example, when one of them dies, it is for the most 

 part torn to pieces, and even the bones destroyed by various 



