246 HABIT AND INTELLIGENCE. [cHAP. 



although the structure formed by their coalescence would be 

 certainly as strong, and perhaps stronger, were it of a single 

 bone, without any vestige of division into parts : at least, 

 we know that in works of engineering it is always best, 

 when it is practicable, to make any part where strength in 

 a small compass is needed, out of a single piece. But even 

 if there is no loss of strength, there cannot be any gain 

 of it by retaining the visible distinctness of the vertebrae. 

 This fact is consequently not explicable as a case of the 

 adaptation of structure to function; and it is a very re- 

 markable case of homological resemblance continued along 

 with, and notwithstanding, adaptive difference. 

 How is How are we to explain these homological relations 1 



t^Tf °^^ Shall we be satisfied with the answer, that it has pleased 

 explained ? the Creator to lay down a plan and keep to it ? This is 

 true : generally true, though not, like the law of gravita- 

 Unity of tion, an invariable truth : but it is no explanation of the 

 no^expla- f^cts ; it is Only a generalized statement of them. Perhaps 

 nation. ^j^ey are inexplicable, though I do not think so. I should 

 not write as I do, were I not convinced that they are 

 capable of explanation to a very great extent : but at least 

 let us not advance that as an explanation which is no ex- 

 planation at all. To say that the wonderful homologies of 

 the vertebrate skeleton, to which I have little ncfore than 

 alluded, exist because the Creator has laid down a plan and 

 adhered to it, is like saying that water rises in a pump 

 beccmse nature abhors a vacuum ; — perfectly true as a 

 statement of facts, utterly unmeaning if offered as an 

 explanation of them. 



Further, it is, I think, a true canon in the logic of 

 No law science, that no law can be an ultimate one which is sub- 

 excepUons ject to limitations or exceptions. By an ultimate law, I 

 can be mean one which is not resolvable into any other. Gravi- 

 tation is such a law, and so, as I believe, are the elementary 

 laws of Habit. But so is not the law that nature abhors 

 a vacuum; and when it was experimentally ascertained 

 that nature abhorred a vacuum to no greater height than 

 about thu-ty-three feet, it became evident, at least to such 

 sound thinkers as Galileo, that the abhorrence of a vacuum 



