Fern, White Blackbird, Drosophila mutations of Morgan, 

 etc. 



Fluctuating Variations vs. Mutations. — It may be in- 

 structive to summarize here the difference between fluctu- 

 ating variations and mutations: — 



(a The former are not transmissible; the latter are : — 

 they breed true. 



(b) The former present no new character, but the same 

 characters differing in degree; the latter bring in a 

 new character, or lose an old one abruptly. 



(c) The former are common; the latter are rare. 



(d) The former permit a series whose differences may 

 be plotted on a frequency curve ; the latter cannot 

 be so plotted, they are discontinuous. 



(c) The former fluctuate about a mean, and never 

 produce a new permanent mean; the latter "cause 

 a new mean to be formed, around which is grouped 

 a new series of fluctuating variations. The real 

 test of difference comes in breeding." 



1. Discuss discontinuous variations in relation to the 

 Biogenetic Law. 



2. Are the gill-slits of embryos of mammal, bird and 

 lizard "embryonic survivals or" "phyletic contrac- 

 tions? 



3. What are chimeras? Vegetative Mutations? Give 

 Examples. 



4. Discuss lethal factors in connection with mutations. 



(b) — Gates' Study of the Chromosomes of Oenotheras 



Gates has recently made a careful study of the chro- 

 mosomes of several species of Oenothera. He finds that 

 Oe. gigas has 28 chromosomes as its diploid number, and as 

 a consequence of the increased amount of chromatin the 

 cells are larger. Oe. lata and scintillans have 15 diploid 

 chromosomes, and as a consequence their meiotic reduct- 

 tion is irregular. Half of the pollen grains and half of the 

 embryo-sacs contain 8 chromosomes, while the other half 

 have 7 chromosomes each. 



These and other facts seem to lead to the conclusion 

 that some of the mutations are due to an irregularity in the 

 distribution of chromosomes, a result of internal accidents. 



Moreover, Gates found that the chromosomes of mu- 

 tation species of Oenothera differ in their shape, size and 

 structure (Gates, R.R. — The Mutation Factor in Evolution, 

 1915). 



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