A man who has no assured and ever present belief in the existence of a personal God or of a future existence with retribution and reward, can have for his rule of life, as far as I can see, only to follow those impulses and instincts which are the strongest or which seem to him the best ones

It's not the strongest, but the most adaptable that survive.

Even people who aren’t geniuses can outthink the rest of mankind if they develop certain thinking habits.

A celebrated author and divine has written to me that he has gradually learned to see that it is just as noble a conception of the Deity to believe that he created a few original forms capable of self-development into other and needful forms, as to believe that he required a fresh act of creation to supply the voids caused by the action of his laws.

There is no fundamental difference between man and animals in their ability to feel pleasure and pain, happiness, and misery.

We have seen that the senses and intuitions, the various emotions and faculties, such as love, memory, attention and curiosity, imitation, reason, etc., of which man boasts, may be found in an incipient, or even sometimes in a well-developed condition, in the lower animals.

The formation of different languages and of distinct species and the proofs that both have been developed through a gradual process, are curiously parallel.