HOMO LOGY. 75 



Spencer does not attempt to apply his previous ex- 

 planation here. He thinks that the segmentation 

 of vertebrates, /. e., the division of the spinal column 

 into vertebrae, arose as the result of strains. Origi- 

 nally the vertebrate was unsegmented, but in bend- 

 ing its body from side to side in locomotion through 

 the water, its spinal column became divided by the 

 action of the simple mechanical force. But with 

 this understanding of the segmentation, the explan- 

 ation of the homology between the leg and arm is 

 lost. Spencer simply says that this homology is 

 due to like conditions acting on the two appendages. 

 But plainly we are no better off than we were to 

 start with, when we remember that similar condi- 

 tions do not always produce the same results, and 

 that almost everywhere can be found instances 

 where the same structure is preserved in spite of 

 different conditions. It is palpable that, when 

 Spencer tries to account for serial homology in ver- 

 tebrates by likeness of conditions, he has simply 

 abandoned the question altogether. 



But, moreover, we have absolute proof that this 

 sort of homology is due to some internal cause rather 

 than external conditions. Many abnormalities of 

 structure show this : instances where extra digits 

 occur on both hands and feet, or the absence of the 

 same digit on both hands and feet ; instances of 

 both hand and foot being similarly affected by 

 peculiar diseases ; the development on the feet of 

 birds of wing feathers like those of the anterior 

 appendage. Enough cases of this kind are known 

 to make it plain that there is some sort of internal 



