SPECULATIVE ZOOLOGY. 369 



ing with the egg, the young entoconcha passes through a series of 

 stages, a, b, c, d, e,f, g, h, which are like stages in the development 

 of an ordinary gasteropod, and therefore like stages in the life of its 

 gasteropod ancestor, A ; hut, after reaching a certain point, it takes 

 the back track shown by the unbroken line, and, gradually losing the 

 structural complexity which has been acquired, becomes an adult, D, 

 which has reproductive organs, but is, in other respects, as unspecial- 

 ized as an ordinary gasteropod at one of its earliest embryonic 

 stages, b. 



It is obvious that paleontology can give us little help in tracing 

 out such a life-history as this, and we turn to the remaining source of 

 evidence, embryology, to examine how far the facts furnished by this 

 department of life-science can afford a basis for phylogenetic general- 

 izations. 



The case which we have just examined shows that the embryology 

 of two related forms may be essentially the same, since both of them 

 have inherited the greater part of their life-history from a common 

 parent, and it would seem at first sight as if all that we need, to enable 

 us to trace out the relationship of all living animals, is a complete 

 acquaintance with the embryology and metamorphosis of each one 

 of them. A comparison of all the stages in the life of one species 

 with all the stages in the life of another species of the same genus 

 ought to show essential identity ; and a comparison of the stages of 

 development of the species of one genus with those of the species of a 

 related genus ought to show how far their history has been the same : 

 the common features in the embryology of two allied families should 

 show how far the history of the species in one of them has been the 

 same as that of the species in the other, and so on, each wider and 

 wider comparison showing broader and broader relationships, until the 

 features which are common to the embryos of all animals unite them 



into one great group. 



As this may be clearer in a more abstract shape, I will try to state 

 it in the form of a diagram (see Fig. 4). 



Suppose that, in studying the development of four species, 1, 2, 3, 

 4, we find that 1 passes through a series of stages, a, b, c, d, e,f, g, h; 

 that 2 presents the series a, b, c, d, e,f, i,j, Tc ; while 3 passes through 

 the stages a, b, c, I, m, n, o, p, q ; and 4 through the stages a, b, c, I, m, 

 r, s, t, u. A comparison of these four life-histories would indicate that 

 their common relationships are such as are represented by the four- 

 branched tree shown in Fig. 4. We have already seen that it is per- 

 fectly possible that n or c or e may not have been an adult animal, 

 but simply a stage in the development of an unknown adult, x ; so 

 there would not be much chance of finding m or e or c as a fossil, and 

 the embryonic record would not show us what the common ancestor of 

 1 and 2 or the more remote ancestor of all four species actually was, 

 but would simply show that they are related in this way ; but it would 

 vol. xxii. 24 



