Page:The Last link.djvu/119

Rh met: for example, the ancestral line of the one-toed digitigrade horse from a four- or five-toed plantigrade and still very generalized Ungulate is approaching completion.

Phylogenetic study has to rely upon other help. This is afforded by comparative anatomy and by the study of ontogeny. If the latter were a faithful, unbroken recapitulation of all the stages through which the ancestors have passed, the whole matter would be very simple; but we know for certain that in the individual development many stages are left out (or, rather, are hurried through, and are so condensed by short-cuts being taken that we cannot observe them), while other features which have been introduced obscure, and occasionally modify beyond recognition, the original course.

Again, the sequence of the appearance of the various organs is frequently upset (heterochronism). Some organs are accelerated in their development, while others,