Chapter 1. This is a very important chapter on methodology in biology, in which Aristotle presents the following central theses:

1. Necessity in Nature is not all alike:
Absolute: relates to eternal things
Hypothetical: relates to things that come to be (things in genesis), in both art and nature

2. The methods (and modes of necessity) of natural science and theoretical science are not the same:
Theoretical science: absolute necessity
Natural science: hypothetical necessity
See Peck's note on this in the Loeb. He refers to Metaphysics 1025bff. Nature operates like craft so natural science, in this context, is like Aristotle's category of productive science.
See also Generation and Corruption 337b25ff.

3. Methodology: Begin with the thing the way it is, and then describe the processes through which it came to be. (Being precedes becoming; the chicken does come before the egg.) The best method of explanation of some feature or part of a man is to say: (i) Because the essence of man is what it is, therefore a man has such and such. (ii) Or, there cannot be a man at all otherwise than with these parts. (iii) Or, lastly, it is well that a man should have them.
Upon these, this follows: Because man is such and such, therefore the process of his formation must of necessity take place in such and such a manner.

DISCUSSION

What belongs in each category? (OR: Which features are hypothetically necessitated by the animal nature as an end?)

• (i) (from Alan Code)
  Blooded or bloodless-part of an animal's being (substance, ousia) Insects are segmented
  Animals have hearts (Animals perceive; the heart is the central organ of perception). (Note: Balme puts hearts in as a type (ii) feature instead.)
  Animals have livers: Animals concoct; the liver is necessary for concoction.
  A certain octopus has one row of suckers due to its length and slimness. (685b15ff)
• (ii) Blooded creatures have bile or a spleen:
  CODE (Kullmann, Sorabji): It is a necessary part, but can't be derived merely from the definition of the thing. This category may fit the type-iii criterion of being "for the best" but it should be understood as the necessary accidents (or kath'hauto sumbebekota). See PA 676b16-677a19, 670z30; also see Sorabji (1980), p. 155.
  GOTTHELF (Cooper, Balme): It is a necessary part given an essential function because it makes some necessary contribution to the performance of that function. See Gotthelf, "First Principles," p. 189 and n. 56.
• (iii) (Gotthelf): The part is not necessary but "at least' best, given an essential function, because it is one among the possible contributors to some necessary function, and the best of that group.
  Example: Having hair on the top of the head.
  Sorabji lists these other references (p. 157, n. 2): PA 670b23 (kidneys), 675a19 (gullets), b23, 654b22 (huckle-bones), GA 717a15-31 (testes). Sorabji says these are examples of "explanation without necessitation."

4. Critique of the "physiologers" (reductive materialists): their statements are unsatisfactory because they leave out the essence and character of the animal and its parts. This essence or form is soul; we must treat the relevant parts of soul (for animals).

5. We should not study nature as an abstraction (i.e. as Platonic Forms). We should study the end (telos) which is prior to the seed and the becoming. There is final cause and also necessity. Here the relevant sort of necessity is conditional or hypothetical; this contrasts to the two modes of philosophical necessity (see Metaphysics 1015a20, absolute and coercive necessity).

Example of respiration:
It takes place for a purpose. (hypothetical necessity)
Each stage in the process follows the other by necessity (material necessity).

Chapter 2. Aristotle argues against (Platonic) division by dichotomy.

Chapter 3. Aristotle argues against (Platonic) division by privations.

Chapter 4. Aristotle argues that we should seek to identify the natural groups, many of which already have been given names.
Among things belonging in one group (genus), members differ by the more and the less.
Among things belonging to different groups (genera), members differ by analogy.

Chapter 5. The rationale for studying "lowly" animals: Aristotle's "pep talk" to philosophy students.