128 CHAPTERS ON EVOLUTION. 



unity in the structure of all vertebrate skeletons. An explanation is 

 required ; we want to comprehend how this unity in diversity has 

 come about. Morphology (the science of structure), studied in the 

 history of embryos, reveals to us an evolution by which the skull 

 passes through one grade of structure after another, becoming 

 advanced and changed by almost imperceptible gradations until the 

 adult type is attained, in a certain number of days and weeks. This 

 evolution is continually going on within our experience; and we 

 little think of its marvels. And yet many find it inconceivable that 

 the same process of evolution can have taken place in past ages, so 

 as to produce from small beginnings the varied fauna of the globe. 

 The natural forces which in a few days," concludes Mr. Parker, 

 " make a chick out of a little protoplasm and a few teaspoonfuls of 

 yolk, are pronounced incompetent to give rise to a slowly changing, 

 gradually developing series of creatures, under changed conditions of 

 life. Yet to our minds the one is as great a marvel as the other ; m 

 fact, both are but the different phases of one history of organic 

 creation." 



The old idea of the " archetype " is thus seen to become resolved 

 into, and to be replaced in time, and through the progress of scientific 

 research, by the primitive form from which all the varied structures 

 of the same kind have arisen by a natural process of evolution. 

 The science of likeness and the theory of development mutually 

 support and confirm each other. No longer do we search for an 

 " archetype " skull or for a typical vertebra. The creative idea in 

 this or in any other department of natural science is not contained 

 in some perfectly formed structure, with all its complexities and 

 intricacies of form already apparent. The true object of our search 

 is for the primitive type ; and the way of our seeking lies through 

 the modifications and paths by which, from that simple type, the 

 abstruse and the complex have been evolved. 



The present is perhaps the most appropriate stage of our inquiries 

 at which to point out that, whilst the broad features of likeness in a 

 series of animals or plants such as those exemplified by the limbs 

 of higher animals are only susceptible of explanation on the theory 

 of evolution, or, in other words, " of inheritance from a common 

 ancestor," there are other features which demand a somewhat 

 different method of treatment. When the subject of homologies is 

 regarded in a broader aspect, we become aware that it is not only 

 possible, but necessary, to regard likenesses from two points of view. 

 The broad homologies of limbs are to be explained, as just remarked, 

 by the theory of descent from a common ancestor. Such structures, 

 the direct product of blood-relationship, are to be called "homo- 

 genous," and illustrate the purest examples of the " likenesses " we 

 are discussing. But it has been already remarked that a law of 



