14 INTRODUCTION. 



ships, and here is a case in point. These Tunicafes, belonging 

 to the division or sub-kingdom Molluscoid^e, were for a time 

 regarded as zoophytes, some of them sociably living, several in a 

 colony, like buds on a stem, and even fixed to one spot (fig. 2). 



Though in their adult state they possess but slight indications 

 of sensibility, they have a heart, respiratory organs, and some of 

 them a single eye in the middle of their transparent body; but in 

 external form they have no resemblance whatever to a vertebrated 

 animal. Some features in their early development, however, give 

 promise of better things. They are at first free-swimming tad- 

 poles, with an indication of backbone ; and when we come to 

 talk about the frog we will give another glance at these Tunicates., 

 which just now appear to be exciting much interest among biolo- 

 gists. A very large collection of them, brought home in the 

 Challenger, after the voyage of 1872-76, has been lately arranged 

 in 102 species, and described for the first time by Prof. Herdman 

 (1886), stimulating researches in various other classes of verte- 

 brates. Not even yet, though specialists have been hard at work 

 in their various branches, are the results of the Challenger expe- 

 dition fully known. Each year new species of minute organisms 

 are described and announced ; almost daily is new light thrown 

 on hitherto doubtful biological questions ; and thus it is that 

 new classifications and additional Sub-kingdoms are so frequently 

 established. 



It has been in order to convey some insight into the intricacies 

 of zoological science, and to introduce the three-eyed lizards, that 

 I have dwelt somewhat on the Invertebrated groups. And there 

 remain a few words still to be added concerning embryology ; the 

 structure of the embryo, or immature animal, being, in the eyes 

 of most living biologists, even more important for classification 

 than that of the adult. Darwin affirmed it to be " one of the most 

 important subjects in the whole round of natural history," 

 because there is a law in embryonic development that — to quote 

 Darwin again — " various parts in the same individual which are 

 exactly alike in the embryonic period, become widely different, 

 and serve for widely different purposes, in the adult state." And 

 it is the early development that indicates the adult relationship, 

 whether to bird, mammal, or fish. This will be evident when, 

 as one of the examples of development, we come to compare the 

 Tunicate tadpole with that of the frog. The eggs of frogs and 

 newts being transparent, the development can be watched from 

 day to day, almost hourly, indeed, in a favourable temperature. 

 And in the whole range of natural history there can scarcely be a 

 more interesting and engrossing study than to contemplate the 



