All known references to the Law of Adaptation (in Time) by Alice A. Bailey (AAB) in her voluminous writings and those given to her by Djwhal Khul (DK). Links on these pages of The Laws of Alice A. Bailey are to the online books at the Lucis Trust website.
What is a law?
… a law is but the spiritual impulse, incentive and life manifestation of that Being in which he lives and moves. A Treatise on White Magic , INTRODUCTORY REMARKS
A law is an expression or manifestation of force applied, under the power of thought, by a thinker or group of thinkers. Discipleship in the New Age, Vol. I , SECTION ONE – DISCIPLESHIP IN THE NEW AGE – Part 2
… a law is in reality the effect of the life of a greater entity as it encloses a lesser within its living processes. Esoteric Healing , Part Three – The Fundamental Laws of Healing
… a law is but the effect of the continued intelligent activity of the Life aspect as it works in conjunction with matter. Esoteric Psychology – Volume I , 5. The Rays in Cyclic Manifestation
A law is only an expression or manifestation of force, applied under the power of thought by a thinker or group of thinkers. Esoteric Psychology – Volume II , II. The Seven Laws of Soul or Group Life – Part 5
A law presupposes a superior being who, gifted with purpose, and aided by intelligence, is so coordinating his forces that a plan is being sequentially and steadily matured. A Treatise on White Magic
Quotes I found below:
This fire is the basic vibration of the little system in which the monad or human spirit is the logos, and it  holds the personality or lower material man in objective manifestation thus permitting the spiritual unit to contact the plane of densest matter. It has its correspondence in the ray of intelligent activity and is controlled by the Law of Economy in one of its subdivisions, the Law of Adaptation in Time.
All these three concepts are governed by the Law of Economy, which is the Law of Adaptation in time and space, or the line of least resistance. This line of least resistance is that which is sought for and followed on the matter side of existence. Incidentally, Brahma manifests Will, because He is purpose, and Love because in this solar system Love is the line of least resistance. While this is an occult statement worthy of consideration, yet it must be remembered that He is primarily activity and intelligence with the aim of adaptability, and that this is His main characteristic.
II. Its subsidiary laws.
1. The law of vibration.
2. The law of adaptation.
3. The law of repulsion.
2. The Law of Adaptation, is the law governing the rotary movement of any atom on every plane and subplane.
The Great Ones look to see the faculty of pliability and adaptability working out, that faculty of adaptation that is one of the fundamental laws of species which nature so wonderfully demonstrates. The transference of this law to the inner planes and its working out in the new cycle of effort must be undertaken. This law of adaptation involves the appreciation of the need, the recognition of the new force coming in with the new cycle and the consequent bringing together in wide synthesis of the need and of the force, regarding the personal self simply as a focal point for action and transmutation.
You are welcome to post your comments and understanding of this law.