Recently many State Boards and CBSE have adopted NCERT books for their board examinations. Keeping in view of this change I have written this textbook for class XII & NEET students. This edition is equally beneficial for class-12 NEET (Medical Exams) CSIR (NET Life Sciences) PGT & other competitive exams. During the writing of this book I always remained conscious of the difficulties of students and have tried to provide simple well-illustrated and up-to-date information on the subject. This book is a compilation from different sources like several modern and classical books including NCERT books. Though I have taken extreme care to give a correct account I am fully aware of my limitations. I always try to redraw the difficult diagrams and figures to simplify the content of the book. If the readers face any inconsistencies the author extended their due apology for that.