REMARKS ON MR. DARWJN'S THEORY. 
135 
4. Why the extinction of old forms is the almost inevitable conse- 
quence of the production of new ones. 
5. Why, when a species disappears, it never re-appears (although 
this is within the range of possibility) . 
6. Why groups of species increase in number slowly, and endure 
for unequal periods of time. 
7. Why, the more ancient a form is, the more it generally differs 
from those now living, 
8. Why all the forms of life are linked together. 
9. Why there is often great difficulty in drawing a line between 
two species. 
10. Why, as a general rule, in life on the globe there have been 
" an ascent, and progress in the main." 
11. Why the lower forms of life have larger specific existences than 
the higher ones*. 
12. Why the older forms lived unchanged for longer periods of 
time than the newer ones, t because they were more widely 
distributed. 
13. Why the deep-sea shells and those of the land and fresh-water 
enjoy a longer range in time than the littoral species ; for 
the iittornl species being confined to narrow zones in depth 
are much more likely to suffer from elevation or subsidence 
than those that live in the deep-sea, or on the land and in fresh- 
water. 
14. Why some animals and plants have rudimentary, and some- 
times useless organs. 
15. Why the homologous parts, so different in the adult, are alike 
in the embrj'o. 
16. Why the embryos of the higher animals resemble, at different 
stages of their existence, the embryos of the lower animals. ;|: 
17. Why "in their infancy the molluscous animals are more alike, 
both in appearance and habits, than in after life.§" 
18. Why the limbs, &c., of aU animals are formed on the same 
plan. 
19. Why the flowers, branches, &c., of plants and trees are but 
rudimentary or metamorphosed leaves. || 
20. Why animals very often resemble in colour and appearance the 
localities which they frequent. 
21. Why in geographical distribution there are generic as well as 
specific centres. 
22. Why typical groups and species are widely distributed, while 
aberrant forms are usually confined to small areas. 
23. Why the inhabitants of islands bear some relation to those of 
the nearest continent. 
* Owen's Palaeontology, p. 49. 
t Anniversaiy Address of Professor Phillips to the Geological Society in Feb. 1860 
X Carpenter's " Principles of Comparative Physiology," p. 95. 
§ Woodward's " Recent and Fossil Shells," p. 10. 
11 Lindley's " Elements of Botany," p. 354. 
