We here see the predominating force of a theory which refuses all possible rationality to any hypothesis but its own. The confident tone with which facts are arrayed and are then pronounced inexplicable upon any other hypothesis than that which the writer asserts, without one scintilla of proof of their tendency to exclude every other supposition, renders the refutation of such reasoning a wearisome task. But there is here one plain and sufficient answer to the whole of the supposed difficulty. The evolution theory, in this particular application of it, is that originally there were vertebrate animals in which the spine consisted of separate movable segments from head to tail, as it does now in fishes and reptiles; but, as the higher vertebrata were evolved out of these lower forms, the movable segments were rendered less movable with respect to each other, and at length in the sacrum the segments became relatively immovable, and yet the originally segmented structure was retained in this part of the column, by force of the propinquity of descent from an antecedent type which had the whole column divided into movable segments. Upon no other hypothesis, it is asserted, is this result explicable. Mr. Spencer's analysis of the sacrum is somewhat defective. It is, as he says, that part of the vertebrate column which in the higher class of vertebrate animals is, during fœtal life, composed, like all the rest of the column, of distinct vertebræ. These vertebræ, like the others, are flexible in the fœtal stage, but after birth they become coalesced or united into one piece, instead of remaining in separate pieces. Thus far, Mr. Spencer's description is, I am informed by anatomists, correct. But the questions which he propounds as if they were unanswerable upon the assumption that this change is inexplicable upon any other hypothesis than that of the evolution of the higher vertebrata out of the lower vertebrate animals, and that the sacrum, with its continuous piece, has retained the segmented outward form by force of the descent, demand closer consideration. Let us trace the process of formation in the human species, and then see what is the just conclusion to be derived from it. In the embryonic condition, the substance which is to form the vertebral column is continuous. As the fœtus is developed, this substance separates itself into the segments which are called vertebræ, and these segments remain flexible and movable throughout the column. After birth, the five lower segments become united in what is substantially one piece, but of course the marks of the original segments remain. This is what occurs in the origin and growth of the individual. Now, looking back to the period when this species of animal did not exist, and supposing it to have been specially created in the two related forms of male and female, endowed with the same process of procreation and gestation that has been going on ever since there is any recorded or