We repeat, then, with additional emphasis after this examination, that we can not imagine between physical and psychical phenomena a relation of cause and effect _in the same sense_ in which we use these terms in physical science, although in some sense there is doubtless such a relation. If man were the only animal we had to deal with, there would be no standing ground left for materialism. But there is still another difficulty which sticks deeper. It is that suggested by the _law of evolution_ and enforced by the _comparative method_. =Relation of Man to Animals.=--Man, we say, is endowed with, _is_, in fact, an immortal spirit. What is spirit? We know things only by their phenomena; what are the phenomena of spirit? Consciousness, will, intelligence, memory, love, hate, fear, desire--surely these are some of them. But has not a dog or a monkey all these? Pressed with this difficulty, some have indeed felt compelled to accord immortal spirit to higher animals. But we can not stop here. If to these, then also to all animals; for we have here only a sliding scale without break. Can we stop now and make it coextensive with sentiency? No; for the lowest animals and lowest plants merge into each other so completely that no one can draw the line between them with certainty. We must extend it to plants also. Shall we stop here and make immortal spirit coextensive with life? We can not; for life-force is certainly correlated with, transmutable into, and derivable from, physical and chemical forces. We must extend it into dead nature also. Therefore, everything is immortal or none. Our boasted immortality by continued extension becomes thinner and thinner until it evaporates into thin air. It becomes naught else than _conservation of energy_, and not, as we had hoped, conservation of _self-conscious personality_. This may be interesting as a scientific fact; but of what value to us personally is a continued existence of our spiritual forces as heat, light, electricity, or any other form of unconscious force? Thus, then, if once we pass the gap between man and the higher animals, there is no possibility of a stopping-place anywhere. Such is the difficulty presented by comparison in the _taxonomic_ series. Take now the _embryonic_ series. Each one of us, individually, was formed gradually by a process of evolution, from a microscopic spherule of protoplasm undistinguishable in structure from the lowest forms of protozoal life. Now, in this gradual process of evolution, where did immortal spirit come in? Was it in the germ-cell? Then why deny it to the protozoan? Was it at the quickening, or at the birth, or at the moment of first self-consciousness, or at some later period of capacity of abstract thought? Again, when it did come in, was it something superadded or did it grow out of something already existing in the embryo or the infant?