EVOLUTION 409 



2. Genetics. 



(a) Inherited changes can always be referred back to ancient Men- 

 delian recessives meeting, and thus producing a "past" type. There can 

 be no strictly "new" types ever formed because the chromosomes never 

 die (as long as there are living offspring), and all that ever happens is 

 that some part of them is thrown out. But, from what is known of 

 biology, it is impossible to add anything to the offspring which is not 

 already present in the chromosome content of the germ cells. 



(b) Mendelian characters themselves are only concerned with 

 minor details such as eye-color, hair-color, and similar matters. No 

 change of a definite survival value has yet been shown to come under its 

 laws. 



3. Comparative Anatomy. 



Similarity of structure by no means proves relationship, as shown 

 by examples of convergent evolution, where two quite dissimilar struc- 

 tures come to look alike in various aspects, due to similar functioning. 

 Witness such experiments as Carey's in which bladder-muscle was con- 

 verted into beating heart-muscle by causing the bladder to simulate 

 heart-conditions. 



The argument from comparative anatomy holds good only if one 

 accept the dictum that "structure determines function," while the experi- 

 ment just mentioned shows that function determines structure, once one 

 has the material with which to work. 



y 



4. Comparative Embryology. 



(a) Any organ not used is likely to degenerate. This accounts for 

 Sacculina degenerating when it assumed a parasitic habit where it no 

 longer uses the various organs it once used. This is not remarkable, and 

 if it proves anything, it proves only that an organism can lose something 

 it once possessed,, though it by no means proves that what we have been 

 considering a more complex organism, can arise from one that is less 

 complex. 



(b) The so-called gill-pouches demonstrate only as in (c) that ver- 

 tebrate forms pass through similar stages of growth and not that one 

 springs from the other. 



(c) If the Haeckelian law is to hold good, that embryos pass 

 through the adult stages of the race to which they belong, we are con- 

 fronted with some unacceptable conclusions. For instance, only the 

 human being walks in an entirely upright position. In man alone there 

 are three complete bends in the developing brain which remain through- 

 out adult life. It is assumed that only his upright position can account 

 for the third bend, which brings the cerebral hemispheres back over the 

 brain-stem. But in the chick, and in practically, if not all, vertebrates, 



