If the continuity of evolution does not warrant us in assigning the same value, æsthetic or moral, to all the links, highest and lowest, in the chain, still less does it authorise or require us to deny all value to the lowest. On the contrary, we should rather see in the lowest what it has of the highest, than look in the highest for the lowest we can find. We should beware lest in reducing everything to its lowest terms we prove to have been seeking simply to bring it to our own level, when at the cost of a little more generosity we might have raised ourselves somewhat nearer to the ideal prefigured even in the lowest stage of the evolution of love, of beauty, of piety or of goodness. Indeed, as a mere matter of logic, it is impossible to state the nature of a cause accurately, quite apart from any question of estimating its value, until or unless we know the effect which it produces. It is not only that we may underrate or entirely overlook the importance of a thing, so long as we are ignorant that it is a factor largely influencing some result in which we are interested; but, until we know what effects it is capable of producing, we do not know what the thing is. We could not be said to have knowledge of a drug if we did not know what its effects were. Nor is that knowledge to be acquired by analysing the causes which produce the drug. It is not from the mechanical causes which give rise to a thing that we can learn what a thing is: no amount of knowledge of the properties of hydrogen and oxygen would enable us to predict _a priori_ the nature of the compound which is formed when electricity is passed through two molecules of the former and one of the latter; nor is the least light thrown upon the properties of water by our knowledge of its constituent elements: on the other hand, our knowledge of them is materially and serviceably increased when we learn what they are capable of producing in combination. We learn most truly what a thing is from observing what it becomes, what use it subserves, what end it answers, what purpose it fulfils--in a word, when we know not its mechanical but its final cause. In biology, knowledge of an organ means knowledge of its function--that is, of its purpose; and evolutional biology also teaches that function is the cause of the organ.